Aluminum cell cathode coating method

ABSTRACT

This invention relates to a method for the application of a coating composition containing Refractory Hard Material to a cathode substrate to prepare an aluminum wettable cathode surface. 
     A mixture of Refractory Hard Material and carbon system is applied to a cathode substrate, cured and carbonized to a non-graphitized carbon matrix containing Refractory Hard Material, characterized by strong bonding of said matrix to said substrate and an ablation rate of said carbon matrix similar to the combined rate of wear and dissolution of the Refractory Hard Material.

BACKGROUND OF THE INVENTION

The manufacture of aluminum is conducted conventionally by theHall-Heroult electrolytic reduction process, whereby aluminum oxide isdissolved in molten cryolite and electrolized at temperatures of from900° C. to 1000° C. This process is conducted in a reduction celltypically comprising a steel shell provided with an insulating lining ofsuitable refractory material, which is in turn provided with a lining ofcarbon which contacts the molten constituents. One or more anodes,typically made of carbon, are connected to the positive pole of a directcurrent source, and suspended within the cell. One or more conductorbars connected to the negative pole of the direct current source areembedded in the carbon cathode substrate comprising the floor of thecell, thus causing the cathode substrate to become cathodic uponapplication of current. If the cathode substrate comprises a carbonlining it typically is constructed from an array of prebaked cathodeblocks, rammed together with a mixture typically of anthracite, coke,and coal tar pitch.

In this conventional design of the Hall-Heroult cell, the moltenaluminum pool or pad formed during electrolysis itself acts as part ofthe cathode system. The life span of the carbon lining or cathodematerial may average three to eight years, but may be shorter underadverse conditions. The deterioration of the carbon lining material isdue to erosion and penetration of electrolyte and liquid aluminum aswell as intercalation by metallic sodium, which causes swelling anddeformation of the carbon blocks and ramming mix.

Difficulties in cell operation have included surface effects on thecarbon cathode beneath the aluminum pool, such as the accumulation ofundissolved material (sludge or muck) which forms insulating regions onthe cell bottom. Penetration of cryolite through the carbon body causesheaving of the cathode blocks. Aluminum penetration to the iron cathodebars results in excessive iron content in the aluminum metal, or in moreserious cases a tap-out. Another serious drawback of the carbon cathodeis its non-wetting by aluminum, necessitating the maintenance of asubstantial height of pool or pad of metal in order to ensure aneffective molten aluminum contact over the cathode surface. One problemof maintaining such an aluminum pool is that electromagnetic forcescreate movements and standing waves in the molten aluminum. To avoidshorting between the metal and the anode, the anode-to-cathode distance(ACD) must be kept at a safe 4 to 6 cms in most designs. For any givencell installation there is a minimum ACD below which there is a seriousloss of current efficiency, due to shorting of the metal (aluminum) padto the anode, resulting from instability of the metal pad, combined withincreased back reaction under highly stirred conditions. The electricalresistance of the inter-electrode distance traversed by the currentthrough the electrolyte causes a voltage drop in the range of 1.4 to 2.7volts, which represents from 30 to 60 percent of the voltage drop in acell, and is the largest single voltage drop in a given cell.

To reduce ACD and associated voltage drop, extensive research usingRefractory Hard Metals (RHM), such as TiB₂, as cathode materials hasbeen carried out since the 1950's. TiB₂ is only very slightly soluble inaluminum, is highly conductive, and is wetted by aluminum. This propertyof wettability allows an aluminum film to be electrolytically depositeddirectly on an RHM cathode surface, and avoids the necessity for analuminum pad. Because titanium diboride and similar Refractory HardMaterials are wetted by aluminum, resist the corrosive environment of areduction cell, and are excellent electrical conductors, numerous celldesigns utilizing Refractory Hard Materials have been proposed in anattempt to save energy, in part by reducing anode-to-cathode distance.

The use of titanium diboride current-conducting elements in electrolyticcells for the production or refining of aluminum is described in thefollowing exemplary U.S. patents: U.S. Pat. Nos. 2,915,442, 3,028,324,3,215,615, 3,314,876, 3,330,756, 3,156,639, 3,274,093, and 3,400,061.Despite the rather extensive effort expended in the past, as indicatedby these and other patents, and the potential advantages of the use oftitanium diboride as a current-conducting element, such compositions donot appear to have been commercially adopted on any significant scale bythe aluminum industry. Lack of acceptance of TiB₂ or RHMcurrent-conducting elements of the prior art is related to their lack ofstability in service in electrolytic reduction cells. It has beenreported that such current-conducting elements fail after relativelyshort periods in service. Such failure has been associated with thepenetration of the self-bonded RHM structure by the electrolyte, and/oraluminum, thereby causing critical weakening with consequent crackingand failure. It is well known that liquid phases penetrating the grainboundaries of solids can have undesirable effects. For example, RHMtiles wherein oxygen impurities tend to segregate along grain boundariesare susceptible to rapid attack by aluminum metal and/or cryolite bath.Prior art techniques to combat TiB₂ tile disintegration in aluminumcells have been to use highly refined TiB₂ powder to make the tile,containing less than 50 ppm oxygen at 3 or 4 times the cost ofcommercially pure TiB₂ powder containing about 3000 ppm oxygen.Moreover, fabrication further increases the cost of such tilessubstantially. However, no cell utilizing TiB₂ tiles is known to haveoperated successfully for extended periods without loss of adhesion ofthe tiles to the cathode, or disintegration of the tiles. Other reasonsproposed for failure of RHM tiles and coatings have been the solubilityof the composition in molten aluminum or molten flux, or the lack ofmechanical strength and resistance to thermal shock. Additionally,different types of TiB₂ coating materials, applied to carbon substrates,have failed due to differential thermal expansion between the titaniumdiboride material and the carbon cathode block. To our knowledge noprior RHM-containing materials have been successfully operated as acommercially employed cathode substrate because of thermal expansionmismatch, bonding problems, etc.

For example, U.S. Pat. No. 3,400,061, of Lewis et al, assigned to KaiserAluminum, teaches a cell construction with a drained and wetted cathode,wherein the Refractory Hard Material cathode surface consists of amixture of Refractory Hard Material, at least 5 percent carbon, andgenerally 10 to 20% by weight pitch binder, baked at 900° C. or more.According to the patent, such a composite cathode has a higher degree ofdimensional stability than previously available. The composite cathodecoating material of this reference may be rammed into place in the cellbottom. This technique has not been widely adopted, however, due tosusceptibility to attack by the electrolytic bath, as taught by a laterKaiser Aluminum U.S. Pat. No. 4,093,524 of Payne.

Said U.S. Pat. No. 4,093,524, of Payne, claims an improved method ofbonding titanium diboride, and other Refractory Hard Materials, to aconductive substrate such as graphite, or to silicon carbide. Thecathode surface is made from titanium diboride tiles, 0.3 to 2.5 cmthick. However, the large differences in thermal expansion coefficientsbetween such Refractory Hard Material tiles and carbon precludes theformation of a bond which will be effective both at room temperature andat operating temperatures of the cell. The bonding is accordingly formedin-situ at the interface between the Refractory Hard Material tile andthe carbon by a reaction between aluminum and carbon to form aluminumcarbide near the cell operating temperature. However, since the bond isnot formed until high temperatures are reached, tiles are easilydisplaced during startup procedures. The bonding is accelerated bypassing electrical current across the surface, resulting in a very thinaluminum carbide bond. However, aluminum and/or electrolyte attack uponthe bond results if the tiles are installed too far apart, and if theplates are installed too close together, they bulge at operatingtemperature, resulting in rapid deterioration of the cell lining and indisturbance of cell operations. Accordingly, this concept has not beenextensively utilized.

Holliday, in U.S. Pat. No. 3,661,736, claims a cheap and dimensionallystable composite cathode for a drained and wetted cell, comprisingparticles or chunks of arc-melted "RHM alloy" embedded in anelectrically conductive matrix. The matrix consists of carbon orgraphite and a powdered filler such as aluminum carbide, titaniumcarbide or titanium nitride. However, in operation of such a cell,electrolyte and/or aluminum attack grain boundaries in the chunks ofarc-melted Refractory Hard Material alloy, as well as the large areas ofcarbon or graphite matrix, at the rate of about one centimeter perannum, leading to early destruction of the cathodic surface.

U.S. Pat. No. 4,308,114, of Das et al, discloses a cathode surfacecomprised of Refractory Hard Material in a graphitic matrix. In thiscase, the Refractory Hard Material is composited with a pitch binder,and subjected to graphitization at 2350° C., or above. Such cathodes aresubject to early failure due to rapid ablation, and possibleintercalation and erosion of the graphite matrix.

In addition to the above patents, a number of other references relate tothe use of titanium diboride in tile form. Titanium diboride tiles ofhigh purity and density have been tested, but they generally exhibitpoor thermal shock resistance and are difficult to bond to carbonsubstrates employed in conventional cells. Mechanisms of de-bonding arebelieved to involve high stresses generated by the thermal expansionmismatch between the titanium diboride and carbon, as well as aluminumpenetration along the interface between the tiles and the adhesiveholding the tiles in place, due to wetting of the bottom surface of thetile by aluminum. In addition to debonding, disintegration of even highpurity tiles may occur due to aluminum penetration of grain boundaries.These problems, coupled with the high cost of the titanium diboridetiles, have discouraged extensive commercial use of titanium diboride inconventional electrolytic cells, and limited its use in new cell design.It is a purpose of the present invention to overcome the deficiencies ofpast attempts to utilize Refractory Hard Materials as a surface coatingfor carbon cathode blocks, and for monolithic cathode surfaces.

SUMMARY OF THE INVENTION

The present invention relates to the application of a carbon-RefractoryHard Material coating composition, containing Refractory Hard Material,a thermosetting binder system, carbonaceous filler and additive, andmodifying agents, to a carbon cathode, which may be cured in-situ to ahard, tough surface comprising RHM in a carbonized binder matrix. Thecarbonaceous additive utilized in this invention may include carbonfibers, which act as crack arrestors and strengtheners. Sufficient RHMis incorporated in the coating material to ensure continuous aluminumwetting of the surface. Thus, the coating method provides an economicand effective means to obtain the advantages of RHM in cathodes, whileeliminating the need for more costly RHM tiles.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to the present invention, it has been found that cathodestructures may be coated with Refractory Hard Material (RHM) combinedwith specified thermosetting resins and other materials to improve theconventional carbon lining of an aluminum reduction cell. Such coatedcathodes combine the advantages of conventional carbon linings, such asstructural integrity and low cost, with desired properties attained byuse of the Refractory Hard Materials. Such improvements includewettability by molten aluminum, low solubility in the moltenaluminum-cryolite environment, good electrical conductivity, anddecreased muck adhesion. In addition, the present invention isapplicable to existing reduction cells without the cost and time of acomplete cell redesign, or the high cost of producing RHM tiles or RHMalloy tiles suggested by the prior art. The coating of the presentinvention may also be used in cell designs which utilize slopedcathodes.

In understanding the concept of the present invention, it is importantthat certain distinctions and definitions be observed. Accordingly, thefollowing definitions shall be applied with respect to this invention.

The "coating composition" used in the method of the present invention iscomprised of Refractory Hard Material, carbonaceous additive,carbonaceous filler, and binder system. As used herein, the terms"coating composition" or "coating material" shall be intended toencompass the combination of all of these materials. The term "coating",on the other hand, may comprise less, depending on state of drying,cure, or carbonization, since for example, mix liquid may be evaporated,and/or polymerized, during cure and carbonization.

The "Refractory Hard Materials" are in general defined as the borides,carbides, silicides, and nitrides of the transition metals in the fourthto sixth group of the periodic system, often referred to as RefractoryHard Metals, and alloys thereof.

"Resinous binder" shall be used to designate a polymerizable and/orcross-linkable thermosetting carbonaceous substance.

The "mix liquid" of the present invention functions in a variety ofmanners in the coating composition of the present invention, dependingupon specific composition. It may be present to allow easy and uniformmixing of the solid components of the coating composition and to providean easily spreadable mass. Certain mix liquids, such as furfural, mayalso permit an increase in the amount of carbonaceous filler which maybe incorporated in the coating composition. The mix liquid may alsoenhance internal bonding and bonding between the coating and the carbonsubstrate, when it is a solvent containing the resinous binder, and/orconstitutes the resinous binder or part of the resinous binder. This isbecause a dissolved resin or liquid resin may more easily penetrate andimpregnate permeable constituents of the coating, as well as the carbonsubstrate. The mix liquid also permits wicking of the resin intointerstitial voids between particles of the coating composition bycapillary action. The mix liquid may act solely as a solvent for theresinous binder (already present in the solids portion of the bindersystem), such as methyl ethyl ketone (which could dissolve a novolac ifpresent in the solids), and be evaporated during cure and carbonizationoperations. If, on the other hand, the mix liquid is present simply asan inert carrier liquid, then it too may be evaporated during cure andcarbonization. Otherwise, the mix liquid may function as a combinedsolvent and resin former, such as furfuryl alcohol and furfural, part ofwhich volatilizes during heating while the remainder become incorporatedinto the resinous binder. In another instance, the mix liquid may be theresinous binder per se, such as where the resinous binder is a liquidsuch as furfural (generally in combination with phenol), furfurylalcohol, or low polymers of these, or a resole. The mix liquid may alsocomprises the resinous binder in the case of a solid resin, such as anovolac, dissolved in a solvent (the solvent portion of which mayvolatilize during heat up), or a high viscosity resin such as apartially polymerized resole thinned by a solvent. The mix liquid mayalso contain gas release agents, modifying agents, and curing agents.

"Binder system" shall be used to indicate resinous binder, mix liquid,and, if required, gas release agents, modifying agents, and curingagents.

"Gas release agent" shall be taken to mean agents present which formliquid phases which seep through the coating and then evaporate, tocreate small channels within the coating to permit release of volatiles.

"Modifying agents" shall be taken to mean materials added to theresinous binder to modify, for example, curing, electrical properties,or physical properties such as flexural strength or impact strengthprior to carbonization of the coating.

"Curing agents" shall be taken to mean agents required to eithercopolymerize with the resin or to activate the resin to a state in whichthe resin may polymerize or copolymerize. Cross-linking or activatingagents fall into this category, as do catalysts required for mostpolymerization and cross-linking reactions.

"Carbonaceous filler" shall be interpreted to mean those carbonaceousmaterials present, either as a component of a known carbon cement or aspart of a proprietary or custom carbon system, having a C:H ratiogreater than 2:1, which are -100 mesh in size. While a carbonaceousfiller may have reactive groups present, and need not be fullycarbonized, such materials do not polymerize with themselves as theresinous binder material does. Further, carbonaceous filler isessentially insoluble in commonly used solvents such as methyl ethylketone or quinoline, while the resinous binder (in its incompletelycured state) is usually soluble therein.

"Carbonaceous additives" shall be those carbonaceous materials present,either as a component of a known carbon cement or as part of aproprietary or custom carbon system, having a C:H ratio greater than2:1, which comprise particulate carbon aggregate having a particle sizerange between -4 mesh and +100 mesh, and/or carbon fibers.

The term "carbon system" shall encompass binder system plus carbonaceousadditive and carbonaceous filler; or, coating composition minus RHM.

"Carbon cement" shall be taken to mean a commercially availablecarbonaceous cement or adhesive, generally comprising a resinous binder,mix liquid, carbonaceous filler, and curing agents, the solid and liquidportions of which may be packaged separately to increase shelf life, oras a premixed cement. Gas release agents, and/or modifying agents may bepresent in such systems, or may be added thereto for use in the presentinvention. Carbonaceous additives are generally added to such systemsfor use in the present invention if not present in the commerciallyavailable formulation.

Pitch may be present as part of the resinous binder, as a modifyingmaterial, but requires the presence of a suitable curing agent, such ashexamethylenetetramine. Such a curing agent may be already present as acomponent of the resinous binder, or may be added thereto to facilitatecross-linkage between the resinous binder and the pitch, or linkagebetween the pitch and carbonaceous filler, or self-linkage between thepolynuclear aromatics which comprise the bulk of pitch. Although pitchis known to constitute a graphite precursor, graphitization is notrealized in the present invention. Thus, the graphite precursor isdispersed within the resinous binder, which is an amorphous carbonprecursor. Pitch may seep through the coating to provide gas releasechannels, and may, in the presence of appropriate curing agents, crosslink with the resinous binder and/or the carbonaceous filler.

The coating material utilized in the present invention is so constitutedto achieve a number of critical objectives. First, the coatingcomposition can yield to accomodate shrinkage and expansion differences,so that the coating adheres tenaciously to the cathode substrate overtemperatures up to about 800° C., at which temperature a slowly heatedcoating composition has been fully cured and carbonized to a rigid mass.It then has a thermal expansion coefficient thereafter very similar tothat of the substrate to which it is applied. Further, the carboncontent, and type of resinous binder, of the coating material are suchthat a high total char formation occurs during carbonization. Thisminimizes formation of large closed voids and excessive gas evolution.Also, the carbon matrix of the carbonized coating has an ablation ratein service equal to or very slightly greater than the combined rate ofwear and dissolution of the Refractory Hard Material in an aluminum cellenvironment, thus assuring even wear of the coating surface, andcontinual exposure of Refractory Hard Material at the surface."Ablation" is defined herein to encompass the loss and consequentthinning of a material through a combination of mechanical and chemicalmechanisms. Purely graphitic structures fail in this respect, due toconsiderably faster loss rates of the anisotropic structure of thegraphitic matrix, caused by sodium intercalation, for example, and theweak bonds between atomic layers of graphitic material. To achieve thesecritical objectives, it has been discovered that it is necessary toprovide a carbon matrix having very specific characteristics, such thatthe carbon matrix of the present invention provides a high degree ofstrength in three dimensions, as opposed to having the planar weaknesspresent in graphite.

First, to achieve an adherent coating over the entire temperature rangeto which the coated cathode block may be subjected, it is critical thatthe coating material exhibit a reasonable degree of dimensional yieldover the temperature range from ambient to about 800° C., in which rangethe coating releases volatiles, and undergoes curing and carbonizationto a solid, rigid mass. In this temperature range, the coatingcomposition may be subject to "thinning" or compression, a verticalthickness change, to accommodate volumetric contraction or shrinkage ofthe coating, vis-a-vis the positive linear expansion of the block towhich it is applied. It is noted that the resinous binder, per se,undergoes considerable expansion and contraction over this temperaturerange, and that the presence of carbon fiber is most effective instrengthening the coating during this stage of cure and carbonization,by minimizing harmful cracking and permitting fine cracking, whichpermits stress relief and helps accommodate expansion differentialsbetween the coating and the substrate. However, once the coatingcomposition has been carbonized to a rigid, hard solid, it is criticalthat the thermal expansion coefficient of the coating be essentiallyequal to the thermal expansion coefficient of the cathode substrate.Thus, the value of the percentage of expansion of the carbonized coatingmust be within about ±0.2 of the value of percentage of expansion of thecathode substrate, and preferably within about ±0.1 over the temperaturerange of from about 800° C. to about 1000° C., for example. Thus, if thecathode substrate to be coated exhibits an expansion of +0.15 percentover the temperature range from about 800° C. to about 1000° C., orhigher, the coating should exhibit an expansion (over the same range) offrom -0.05 to +0.35 percent, and preferably from +0.05 to +0.25 percent.

It is desirable that the amount of shrinkage that the cured bindersystem undergoes during carbonization be as small as possible. This maybe accomplished by selection of a carbonaceous resin which when utilizedin accordance with the present invention will provide a coatingcomposition which when subjected to carbonization exhibits a shrinkageof the coating on the substrate less than that which would inducecoating failure, cathode block failure, or separation of coating fromthe cathode substrate. Fine vertical cracking within the carbonizedcoating is an acceptable stress relief mechanism. The presence ofcarbonaceous additive and/or filleris beneficial.

Both shrinkage and expansion of the cured binder system and the coatingcomposition may be measured in the following fashion. A sample of thecomposition to be tested is prepared and spread in a Teflon® coated moldhaving dimensions 5 cm×1.27 cm×0.64 cm deep. The composition is cured,and allowed to cool before removal from the mold. The piece is then cutinto four test samples 2.54 cm×0.64 cm×0.64 cm, which are then dried toconstant weight at 250° C. in an alumina crucible. A test sample ismeasured utilizing a micrometer, then heated from room temperature to1000° C. in a dilatometer which is continuously flushed with argon. Theexpansion/contraction is recorded continuously as a function oftemperature on a chart recorder. Two expansion/shrinkage values arecalculated: One based on original sample length and final length at1000° C.; the other based upon original sample length and final lengthafter return to room temperature (this is termed "overall contraction").Samples are also measured with the micrometer after cooling, as a checkon the chart recorder. It is desired that the full cycle or overallcontraction be preferably less than about 1.0 percent.

In addition to the above considerations, it has been found critical toutilize a binder system which, when subjected to carbonization, has achar yield of greater than about 25 percent. Char yield is definedherein as the mass of stable carbonaceous residue formed by the thermaldecomposition of unit mass of the binder system. Thermogravimetricanalyses of various binder system have demonstrated that the amount ofchar yield is a function of the aromaticity of the resin structure. Ingeneral, carbon rings that are bonded at two or more sites will usuallyremain as char. Ladder polymers are the most stable, losing onlyhydrogen, and giving a very high carbon char yield.

Char yield of a binder system, as utilized herein, is determined bycuring a proposed carbon system (i.e. binder system plus carbonaceousfiller) for a 24 hour period so as to achieve polymerization and/orcross-linkage, followed by heating at 250° C. for sufficient time toachieve constant weight, so as to eliminate volatiles, polymerizationproducts, and/or unreacted liquid. The sample is then sintered to 1000°C. in a non-oxidizing atmosphere, and the remaining char weightdetermined. Similarly, the char weight of carbonaceous filler present inthe carbon system is determined, and subtracted from the char weight ofthe carbon system to determine the char weight of the binder system.From the weight of the carbon system at 250° C., and the known weight ofcarbonaceous filler at 250° C., one may calculate the weight of thebinder system at 250° C. The char yield of the binder system is thencalculated, as a percentage, from the char weight of the binder systemand the weight of the binder system at 250° C. It has been observed thatbinder systems exhibiting a char yield of greater than about 25% giveacceptable coatings upon cure and carbonization, while a binder systemexhibiting 8% char yield gave an unacceptable carbon matrix uponcarbonization. Char yields in excess of 50% are preferred.

To achieve a long-lasting coating in the environment of an aluminumcell, it is desired that the rate of ablation of the cured andcarbonized carbon system be close to that of the Refractory HardMaterial in such environment. As the Refractory Hard Material is removedfrom the coating, the carbon matrix thereof is removed at a similar orvery slightly faster rate, thus exposing additional Refractory HardMaterial to the cell environment. In this manner, the coated cathodesurface remains essentially constant, in terms of Refractory HardMaterial content, thus improving cell operation as measured byuniformity of performance. In previous attempts to provide RefractoryHard Material coated cathodes, ablation and/or intergranular attack haveresulted in rapid surface deterioration due to depletion of either theRefractory Hard Material or the carbon matrix at a rate greater than theother, resulting in periods when there are localized areas having eithera Refractory Hard Material-rich surface composition with insufficientbinding capability, or a carbon-rich surface with insufficientRefractory Hard Material. The present invention overcomes these failuresby providing a coating in which Refractory Hard Material and carbonmatrix are dissolved or otherwise depleted at approximately equal rates.

It is important to clarify or distinguish between carbonization andgraphitizing as they apply to heating carbonaceous bodies in the contextof the present invention. "Carbonizing" is normally done by heating acarbonaceous body, either in unitary or particulate form, for thepurpose of driving off volatiles, and progressively increasing the ratioof carbon to hydrogen, and to progressively eliminate hydrogen from thebody. In the carbonizing process, temperature is gradually increased toallow for the slow evolution of volatiles such as decomposition productsso as to avoid blister formation, and to permit volumetric shrinkage(which will occur at some point in the operation) to proceed gradually,so as to avoid formation of large cracks. While curing is considered totake place at temperatures up to about 250° C., carbonizationtemperatures normally range from about 250° C. to about 1000° C.,although higher temperatures up to 1600° C. or higher also can beemployed. While carbonization may be continued to about 1000° C., orhigher, the carbonization of the carbonaceous materials present isessentially complete at about 800° C., and the resinous binder has beencarbonized to bind the filler materials and RHM into a durablestructure. The initial curing and initial stage of the carbonizationoperation are normally carried out in a conventional radiant orconvection-type furnace heated by gas or oil, with the heat input to thecarbon being by indirect heat transfer, or direct flame contact. At somepoint, e.g. above about 250° C., the carbon body becomes sufficientlyelectrically conductive to permit resistive heating, if desired.

It is to be noted that in the present invention, the coating material ispreferably in the form of a workable paste, which is trowelled orsmoothed to a desired thickness and surface smoothness. This coating iscured, by polymerizing and/or cross-linking, and losing volatiles, byslowly heating to about 250° C., at which point the coating has reachedthe thermoset stage and formed a relatively solid mass. Carbonizing, attemperatures above 250° C., then converts this coating to a rigid matrixconsisting essentially of non-graphitized carbon with RHM, carbonaceousadditive, and carbonaceous filler therein.

A distinction is to be drawn between this process and prior artprocesses employing only pitch binders, in the absence of cross-linkingor polymerizing agents. By itself, pitch does not cross-link, orpolymerize, but in fact passes through a liquid "plastic" state or zonebetween about 50° C. and about 500° C., in which temperature rangesubstantial swelling occurs, succeeded by a period when the carbonaceousmass congeals (and contracts) into a hard solid coke body. The coatingcomposition used in the present invention, on the other hand, commencespolymerization and/or cross-linking at temperatures which may be as lowas about 250° C., and is cured to a relatively hard resin state bytemperatures below about 250° C., dependent upon cure time and coatingthickness, followed by progressive hardening through carbonization.

The entire heating cycle in carbonization is somewhat time consuming.Carbonizing typically results in loss of volatiles, and elimination ofvolatile reaction products of thermal decomposition. However, there isno significant change in the crystallographic structure of thecarbonaceous additive or filler, and the carbonized resin continues toappear as amorphous, even though it may bond together a substantialquantity of graphitic material or material containing graphiticcrystallites.

Graphitization is readily distinguished from carbonizing orcarbonization, as described, in that it requires considerably highertemperatures and longer time periods, and produces drastic and easilyobserved changes in the atomic structure. In graphitizing, thetemperatures employed range from a little over about 2000° C. up to3000° C., with the more typical temperatures ranging from about 2400° C.or 2500° C. to 3000° C., as these temperatures are usually associatedwith the higher quality grades of graphite. This heating occurs over arather extensive time period, typically about two weeks. The heating isdone in a non-oxidizing atmosphere, typically by passing electriccurrent directly through the carbon so as to heat the carbon internallyand directly by its own electrical resistance, as opposed to the moreconventional furnace and heating means employed in carbonizing.Graphitizing drastically alters and rearranges an amorphous or partiallygraphitic internal structure, by developing a graphite crystal atomicarrangement. A graphite structure exhibits the well-known close packed,layered, and specifically oriented graphitic structural arrangement.Generally, graphitization is only practicable with the well knowngraphite precursor substances such as pitch.

To illustrate some of the differences in internal structure in comparinggraphite with non-graphitic or amorphous carbon, the d₀₀₂ and L_(c)dimensions are useful. The "Lc" dimension applies to the crystal orcrystallite size in the "c" direction, the direction normal to the basalplane, and the "d₀₀₂ " dimension is the interlayer spacing. Thesedimensions are normally determined by x-ray diffraction techniques. R.E. Franklin defines amorphous carbon as having an interlayer spacing(d₀₀₂) of 3.44Å and crystalline graphite of 3.35Å. (ActaCrystallographica, Vol. 3, p. 107 (1950); Proceedings of the RoyalSociety of London, Vol. A209, p. 196 (1951); Acta Crystallographica,Vol. 4, p. 253 (1951).) During the process of graphitization, theamorphous structure of graphite precursor carbons is changed to thecrystalline structure of graphite which is shown by an increase in theL_(c) dimension and a decrease in the d₀₀₂ dimension. In amorphouscarbon, the L_(c) dimension normally ranges from about 10 to about 100Angstrom units (Å), whereas most graphite typically exhibits an L_(c)dimension of greater than about 350 or 400Å, typically from over 400Å toabout 1000Å. There is another substantially graphitic structure whereinL_(c) is between about 100Å or more up to about 350 or 400Å, and this issometimes referred to as "semi-graphitic", having the same generalatomic arrangement and configuration in its structure as graphite justdescribed but differing some from the more common x-ray diffractionpattern for graphite due to a slight difference of orientation ofsuccessive atomic planes. Both graphite structures have a d₀₀₂ dimensionless than about 3.4Å. In general, graphitizing at temperatures fromabout 2000° C. up to about 2400° C. tends to produce the"semi-graphitic" structure whereas temperatures over 2400° C. tend toproduce the "normal" graphite structure.

One acceptable practice in producing carbonaceous coatings according tothe present invention is to employ particulate graphite as a fillermaterial which is added to the binder and other components. The mixtureis then spread, cured, and carbonized. While this carbonizedcarbonaceous material may contain some graphite, it is not bonded by thegraphite, but rather contains both graphite particles from the fillerand amorphous carbon derived from the binder and/or components of thecarbonaceous filler. In practicing the present invention it is importantthat the carbonized cathode coating be constituted of a non-graphitizingbinder so as to assure the proper combination of electrical and thermalconductivity, coefficient of expansion, ablation rate, and stabilityproperties in the carbon-Refractory Hard Metal surface.

While the borides, carbides, silicides and nitrides of elements inGroups IV to VI of the Periodic Table generally all possess high meltingpoints and hardness, good electronic and thermal conductivity, arewetted by molten aluminum, and are resistant to aluminum andalumina-cryolite melts, TiB₂ is the preferred RHM due to its relativelylow cost and high resistance to oxy-fluoride melts and molten aluminum.Suitably, Refractory Hard Material particle sizes may range fromsubmicron to about 10 mesh, and preferably from submicron to about -100mesh, and most preferably about -325 mesh.

It is generaly thought that grain boundaries between (TiB₂ or otherRefractory Hard Material) crystals are sensitized, i.e., are madesusceptible to attack by the segregation of oxide impurities. Migrationof oxygen dissolved in individual crystals of TiB₂ to crystal boundariesis also thought to be possible by diffusional processes at thetemperatures at which aluminum cells normally run, i.e., around 1000° C.Thus, oxygen impurities, whether or not originally segregated at thesurface of TiB₂ crystals, can migrate to the inter-crystallineboundaries and make them susceptible to attack. Attack of the entirearea of the intercrystalline boundaries results in loss of TiB₂crystals.

It has now been shown that single crystals of TiB₂ bicrystals of TiB₂,open clusters of crystals of TiB₂, and/or crushed and ground crystals ofTiB₂, when in intimate contact with the carbon matrix do not crack ordisintegrate, when exposed to bath and molten aluminum in aluminum cellsfor long periods of time. All of these shall be included within the term"single crystals" as employed herein. It is also to be understood thatwhile discussion focuses on TiB₂, other RHM materials are also intended.The common feature of all of these particles is that they generally havemore exposed free crystal surfaces or transcrystalline fractures thaninternal grain boundaries between two or more crystals. The bindersystem is thus able to adhere to almost every TiB₂ crystal, throughbonding to crystal facets or broken surfaces. The open clusters ofsmaller TiB₂ crystals can be penetrated by the carbonaceous bindersystem, so that even if the grain boundaries between TiB₂ crystalsshould be attacked by the bath, the TiB₂ crystals will still be heldwithin the coating by the binder. This has been observed in brokenpieces of the carbonized coating. TiB₂ tends to break in a conchoidal(or glasslike) manner so that broken pieces have surfaces that cutacross crystal planes, and the binder system forms strong bonds to suchsurfaces.

Both the carbon and TiB₂ components of surfaces made in accordance withthis invention appear to slowly dissolve, or erode, so that new TiB₂crystals are continuously exposed to the molten aluminum. Both TiB₂ andcarbon dissolve through various chemical mechanisms in the aluminummetal in the cell up to saturation concentrations. However, they may bepresent beyond the saturation limit as undissolved particles of theelements, or in compounds containing the elements. The rate at whichboth TiB₂ and carbon are lost from the cathode surface is dependent onsuch factors as temperature, motion of the metal, bath, and muck, andoverall bath chemistry. It is believed that some of the TiB₂ particlesare carried away into the aluminum before being completely dissolved.The TiB₂ concentration in the aluminum metal typically is found to beonly slightly above the solubility limit for the temperature at whichthe cell is operated. The rate of carbon loss is a critical factor inRHM-containing wetted cathode cells as the carbon is used to bond theRHM particles in the cathode coating. An excessive rate of carbon lossresults in undercutting TiB₂ particles in the matrix, and a subsequentincrease in TiB₂ loss from the cathode surface. Excessively low carbonloss would result in areas of local depletion of TiB₂ particleconcentration at the cathode surface, and subsequently a loss ofaluminum wetting.

The TiB₂ preferred for use in this invention is typically specified as-325 mesh. If the TiB₂ is made by carbothermic reduction on titanium andboron oxides and carbides, individual particles will normally fit theprerequisite category of single crystals. This also holds true for TiB₂made by plasma methods described in U.S. Pat. No. 4,282,195 to Hoekje ofPPG Industries. Producing TiB₂ by an arc melting process generallyresults in a polycrystal comprised of relatively large individualcrystals. Hence, on crushing to -20 mesh, this would normally provideparticles that each have only short sections of internal grain boundary.If -325 mesh TiB₂ powder is made by crushing arc-melted TiB₂ chunks,each of the resulting particles is smaller than the original individualcrystals, so that each particle consists of a broken piece of a largercrystal (or, for example, 2 or 3 crystals joined together by someinter-crystalline boundaries). Thus, very few crystals would be entirelyisolated from the binder system, and even if the few inter-crystallineboundaries are completely dissolved by the bath, very few crystals ofTiB₂ would break away.

TiB₂ made by carbothermic reduction of powders is typically composed ofmaterial that is easily crushed to -325 mesh without breaking many ofthe individual crystals. This material, when viewed in a scanningelectron microscope, is seen to be composed of tiny single crystals,open clusters of TiB₂ single crystals, and broken pieces of TiB₂ singlecrystals.

Other RHM materials may be successfully substituted for TiB₂ in thecoatings disclosed herein, when appropriate changes in the coatingcomposition are made to account for differences in wettability, surfacearea, particle size, porosity, and solubility of the RHM. Sufficient RHMis incorporated in the coating composition to ensure aluminum wetting,while thermal expansion mismatch effects are minimized and a dissolutionrate of Refractory Hard Material less than the rate of loss of thecarbon matrix of the coating is achieved. While discussion of theinvention will focus on the use of TiB₂ as the preferred RHM, it iscontemplated that any suitable RHM, such as ZrB₂, or alloys ofRefractory Hard Materials, may be utilized. Sufficient RHM is providedin the coating composition to ensure wettability. In general, the RHMmay comprise from about 10 to about 90 percent by weight of the coatingcomposition, and preferably from about 20 to about 70 percent. It hasbeen found that wettability may be achieved at concentrations belowabout 10 percent, but better results are achieved at ranges from 20percent upwards, with from about 35 to about 60 percent being the mostpreferred range.

The resinous binders utilized in the present invention may comprise anywhich meet the aforementioned criteria. Typical resins which can beemployed include phenolic, furane, polyphenylene, heterocyclic resins,epoxy, silicone, alkyd, and polyimide resins. Examples of phenolicresins which can be employed incude phenol formaldehyde, phenolacetaldehyde, phenol-furfural, m-cresolformaldehyde andresorcinolformaldehyde resins. Epoxy resins which can be utilizedinclude the diglycidyl ether of bisphenol A, diglycidyl ether oftetrachlorobisphenol A, diglycidyl ether of resorcinol, and the like,and especially the epoxy novolacs. Preferred epoxies comprise theglycidyl ethers such as the glycidyl ethers of the phenols, andparticularly those prepared by reacting a dihydric phenol withepichlorhydrin, e.g., the diglycidyl ether of bisphenol A, and epoxynovolacs. The silicone polymers which can be employed include methylsiloxane polymers and mixed methyl phenyl siloxane polymers, e.g.,polymers of dimethyl siloxane, polymers of phenylmethylsiloxane,copolymers of phenylmethylsiloxane and dimethylsiloxane, and copolymersof diphenylsiloxane and dimethylsiloxane. Examples of heterocyclicresins are polybenzimidazoles, polyquinoxalines and pyrrones. Any of thewell known specific alkyds, particularly those modified with phenolformaldehyde, and polyimide resins can be employed. The phenolics andfuranes are the preferred class of resins, particularly in view ofrelatively low costs.

Furane resins are very advantageously employed as the resinous binderused in this invention. Acids and bases commonly are used as catalystsfor furane resin polymerization, but may not be required as furanes maycopolymerize with other resin in the absence of catalysts. Suitable acidcatalysts include inorganic and organic acids such as hydrochloric acid,sulfuric acid, nitric acid, orthophosphoric acid, benzene sulfonic acid,toluene sulfonic acid, naphthalene sulfonic acid, maleic acid, malonicacid, phthalic acid, lactic acid, and citric acid. This family ofcatalysts also includes organic anhydrides such as maleic anhydride andphthalic anhydride. Examples of further satisfactory conventional acidcatalysts include mineral acid salts of urea, thiourea, substitutedureas such as methyl urea and phenyl thiourea; mineral salts of ethanolamines such as mono-, di-, and triethanolamine; and mineral acid saltsof amines such as methyl amine, trimethyl amine, aniline, benzyl amine,morpholine, etc. Preferred acid catalysts have a K_(a) of at least 10⁻³.

Suitable basic catalysts include alkali hydroxides such as lithiumhydroxide, sodium hydroxide, potassium hydroxide, and the like; andalkaline earth hydroxides such as magnesium hydroxide, calciumhydroxide, and the like. Other satisfactory base catalysts include forexample, amine catalysts such as primary amines like ethyl amine, propylamine, etc.; secondary amines like diisopropyl amine, dimethyl amine,etc.; and tertiary amines like triisobutylamine, triethylamine, etc.Examples of other satisfactory base catalysts are mixtures of alkalihydroxides and alkaline earth hydroxides such as mixtures of sodiumhydroxide and calcium hydroxide. A mixture of alkali hydroxides andalkaline earth hydroxides provides a thermosetting binder givingunexpectedly high carbon yields. Suitable amounts of catalyst or curingagents are included with the resinous binder, dependent upon thespecific binder selected. Determination of the appropriate curing agent,and the appropriate amount thereof, are within the skill of the art.

By the term "acid cured furane resins" is meant resins such ashomopolymers of furfuryl alcohol, homopolymers of furfuryl alcoholcross-linked with furfural, copolymers of furfuryl alcohol andformaldehyde, copolymers of furfuryl alcohol and phenol, copolymers offurfural and phenol, or monomeric materials which contain the furanering somewhere in the structure, and are capable of being cured to afinal set and hardened mass by the addition thereto of an acid catalyst.

Resinification of the compositions in question is dependent uponhydrogen ion concentration and is accelerated by heating, as is wellknown. Because the reaction occasioned by the addition of the acidiccatalyst to the resin is exothermic, care must be taken in the selectionand amount of catalyst used, otherwise resinification may proceed toorapidly and the resultant mass may be useless. For this reason,inorganic acids such as hydrochloric acid, sulfuric acid, phosphoricacid and chromic acid generally are not used. On the other hand,inorganic acid salts such as zinc chloride, sodium bisulfate andmercuric chloride are often used, as are latent acid catalysts such asmaleic anhydride and phthalic anhydride, which only on heating yield thenecessary acid catalyst.

In general, organic compounds ordinarily are preferred to achieve thedesired properties in acid cured furane resinous products. In additionto the aromatic sulfonic acids, the aromatic sulphonochlorides such asbenzene sulphonochloride, peraacetyl benzene sulphonochloride; thealiphatic amino salts of the aromatic sulphonic acids, including theammonium salts such as ammonium paratoluene sulphonate, dimethyl aminobenzene sulphonate, diethyl amino toluene sulphonate, ammonium benzenesulphonate and disulphonate, ammonium phenol sulphonate, ammoniumnaphthalene sulphonate and disulphonate, ammonium anthracene sulphonateand disulphonate, ammonium sulphanilate; the aromatic amino salts ofaromatic sulphonic acids such as the aniline salt of benzene sulphonicacid, the aniline salt of paratoluene sulphonic acid, and the pyridinesalt of phenol sulphonic acid; the organic salts of strong inorganicacids such as glyoxal sulphate; the metallic salts of chlorosulphonicacid such as sodium chlorosulphonate and potassium chlorosulphonate; thealiphatic and aromatic salts of strong inorganic acids such astriethanolamine chloride, aniline hydrochloride, ammonium sulphamate,pyridine sulphate, pyridine bisulphate, and aniline sulphate; the aminosalts of sulphanilic acid such as aniline sulphanilate and pyridinesulphonate; the ferric salts of sulphonic acids such as ferrictrichlorobenzene; acid anhydrides such as phosphoric anhydride andmaleic anhydride; the ammonium salts of alkane sulphonic acids such asammonium ethane sulphonate; the ferric salts of sulphonic acids such asferric benzene sulphonate and ferric toluene sulphonate; and theammonium salts of organic substituted inorganic acids such as ammoniumethyl phosphate, have been used as catalysts for furane resins.

Alkaline catalysed furane binders suitable for practice of the presentmethod include furfural plus a phenol, furfural plus a ketone, andfurfuryl alcohol plus an aldehyde and an amine. The furane binder mayoptionally be mixed with pitch, to form a binder material. This bindermaterial may be mixed with a carbon aggregate or filler. Pitches, suchas coal tar pitches, may be present as a modifying component of thebinder system, and high boiling point condensed aromatic systems fromthose are frequently found as impurities in filler materials derivedfrom pitch. Furane binders as contemplated herein, suitable for use inthe present invention, include the following (with appropriatecatalysts):

a. furfural plus a phenol; specific example: furfural and phenol;

b. furfural plus a ketone; examples: furfural and acetone, or furfuraland cyclohexanone;

c. furfuryl alcohol plus an aldehyde and an amine; specific example:furfuryl alcohol and formaldehyde and urea.

One example of a furfural binder suitable for use in the presentinvention, which is relatively fluid at room temperature and has asuitable char yield, is a mixture consisting essentially of between 50and 75 percent by weight of coal tar pitch having a softening pointabove 100° C., and between 50 and 25 percent by weight of monomericpolymerizable thermosetting dispersants consisting of furfural and amember selected from the group consisting of phenol, cyclohexanone, andcompounds having the formula:

    CH.sub.3 -CO-R

wherein R is a hydrocarbon group having between 2 and 4 carbon atoms,inclusive, and a catalyst. The hydrocarbon group in the above formulamay be saturated or unsaturated and straight or branched chain. To thisbinder may then be added appropriate amounts of RHM and carbonaceousfillers to obtain an effective coating composition. The time andtemperature necessary to solubilize the coal tar pitch in thedispersants will vary with the softening point of the coal tar pitch,the total amount of dispersants, and the relative amounts of the severaldispersants. Excessive heats should be avoided to prevent prematurepolymerization of the binder. While the parameters of time andtemperature necessary to solubilize the coal tar pitch in thedispersants cannot be generally stated, they are easily determined byone skilled in the art. In this type of coating composition, the pitchdoes not function as a modifying agent for the binder, but rather as afunctional ingredient having a desirably high char yield.

Generally speaking, molar ratios of furfural to phenol between 0.5 and 2moles of furfural to 1 mole of phenol are preferred when phenol is thesecond ingredient of the dispersant component; and molar ratios between1 and 2 moles of furfural to 1 mole of cyclohexanone, methyl aliphaticketone of the above formula, or mixtures thereof are preferred whencyclohexanone, methyl aliphatic ketone, or mixtures thereof are thesecond ingredient of the dispersant component.

One currently commercially available alkaline-catalyzable furane binder,having utility in the present invention, is that obtainable from QuakerOats Company under the commercial or trade designation "QX-362". It ispresently believed that the principal constituents of this binder arefurfural and cyclohexanone, although some furfuryl alcohol may also bepresent.

A group of furfuryl resins which is considered suitable in accordancewith the invention is furfuryl alcohol copolymers made by reactingmaleic acid or maleic anhydride with a polyhydroxy compound such asethylene glycol. This forms an ethylenically unsaturated, polycarboxylicacid ester prepolymer. The ester prepolymer is then copolymerized withfurfuryl alcohol to produce the furfuryl alcohol copolymer. The furfurylalcohol copolymers described are advantageous because of certainproperties such as relatively low volatility, ease of storage, givingoff of a minimum of water upon curing, resistance to excessive shrinkageand a relatively short cure reaction. Such copolymers may also be quitesuitable for use in accordance with the present invention because ofother properties, namely their ability to remain highly flexible aftercuring.

In addition to those set forth as components of the commerciallyavailable carbon cements, such as UCAR® C-34, discussed hereinafter, awide variety of novolac resins may be used as the basic resinous binder.The term novolac refers to a condensation product of a phenolic compoundwith an aldehyde, the condensation being carried out in the presence ofan acid catalyst and generally with a molar excess of phenolic compoundto form a novolac resin wherein there are virtually no methylol groupssuch as are present in resoles, and wherein the molecules of thephenolic compounds are linked together by a methylene group. Thephenolic compound may be phenol, or phenol wherein one or more hydrogensare replaced by any of various substituents attached to the benzenering, a few examples of which are the cresoles, phenyl phenols,3,5-dialkylphenols, chlorophenols, resorcinol, hydroquinone, xylenols,and the like. The phenolic compound may instead be naphthyl orhydroxyphenanthrene or another hydroxyl derivative of a compound havinga condensed ring system. It should be noted that the novolac resins arenot heat curable per se. Novolac resins are cured in the presence ofcuring agents such as formaldehyde with a base catalyst,hexamethylenetetramine, paraformaldehyde with a base catalyst,ethylenediamineformaldehyde, and the like.

For purposes of the present invention, any fusible novolac which iscapable of further polymerization with a suitable aldehyde may beemployed. Stated another way, the novolac molecules should have two ormore available sites for further polymerization and/or cross-linkage.Apart from this limitation, any novolac might be employed, includingmodified novolacs, i.e., those in which a non-phenolic compound is alsoincluded in the molecule, such as the diphenyl oxide or bisphenol-Amodified phenol formaldehyde novolac. Mixtures of novolacs may beemployed or novolacs containing more than one species of phenoliccompounds may be employed.

Novolacs generally have a number-average molecular weight in the rangefrom about 500 to about 1,200, although in exceptional cases themolecular weight may be as low as 300 or as high as 2,000 or more.Unmodified phenol formaldehyde novolacs usually have a number-averageweight in the range from about 500 to about 900, most of thecommercially available materials falling within this range.

Preferably, novolacs with a molecular weight from about 500 to about1,200 are employed in the present invention. When a very low molecularweight novolac is used, the temperature at which such novolacs softenand become tacky is usually comparatively low.

A mixture of pitch and novolac may be formed by any convenient techniquesuch as dry blending or melting the pitch and novolac by heatingtogether to form a homogeneous mixture. Various pitches may be utilizedfor this purpose, as previously indicated.

In addition to the novolac phenolic resins, the use of resoles is alsowithin the scope of the present invention.

Resole resins are most frequently produced by the condensation ofphenols and aldehydes under alkaline conditions. Resoles differ fromnovolacs in that polynuclear methylol-substituted phenols are formed asintermediates in resoles, and resoles are produced by reaction ofphenolic substances with excess aldehyde in the presence of an alkalinecatalyst. A resole produced by the condensation of phenol withformaldehyde most likely proceeds through intermediates having thefollowing type of structure: ##STR1##

In a typical synthesis, resoles are prepared by heating one mole ofphenol with 1.5 moles of formaldehyde under alkaline conditions.

The resole resins are prepared by the condensation of phenols withformaldehyde, or more generally, by the reaction of a phenolic compound,having two or three reactive aromatic ring hydrogen positions, with analdehyde or aldehyde-liberating or engendering compound capable ofundergoing phenol-aldehyde condensation. Illustrative of phenols arephenol, cresol, xylenol, alkyl phenols such as ethylphenol, butylphenl,nonylphenol, dodecylphenol, isopropylmethoxyphenol, chlorophenl,resorcinol, hydroquinone, naphthol, 2,2-bis(p-hydroxyphenyl)propane, andthe like and mixtures of such phenols. Large aliphatic groupssubstituted on the benzene ring may detract from the present invention,since these are lost in heating and hence may decrease char yield andincrease volatile emission. Illustrative of aldehydes are formaldehyde,paraformaldehyde, acetaldehyde, acrolein, crotonaldehyde, furfural, andthe like. Illustrative of the aldehyde engendering agents ishexamethylenetetramine. Ketones such as acetone are also capable ofcondensing with the phenolic compounds to form phenolic resins.

The condensation of a phenol and an aldehyde is conducted in thepresence of alkaline reagents such as sodium carbonate, sodium acetate,sodium hydroxide, ammonium hydroxide, and the like. When thecondensation reaction is completed, if desired, the water and othervolatile materials can be removed by distillation, and the catalystneutralized.

The resole reins are termed heat curable resins. That is, under theapplication of heat these resins progressive polymerize until they arefinally insoluble, infusible and completely cured. For the purposes ofthe present invention, the curable phenolic resins are considered thosewhich have not so advanced in polymerization that they have becomeinfusible.

Furfuryl alcohol may be employed as the mix liquid in the phenoliccarbonaceous binder, and is believed to react with the phenolic resin asit cures, and serves as a modifying agent for the resin. The use offurfuryl alcohol is preferred as it has been found that bonds having thehigh strength obtainable through the use of this mix liquid cannot beproduced when other mix liquids are substituted for furfuryl alcohol.Thus, for example, when furfuraldehyde is employed in place of furfurylalcohol in otherwise identical compositions, bonds are produced havingonly about half the strength of the bonds produced using the furfurylalcohol.

Since the net final effect desired is to achieve a surface layercomposed essentially of RHM and carbon, the binder system should bereadily decomposable, in high yield, to a carbon residue. Suchcomponents as resinous binder should comprise from about 1 to about 40percent of the coating composition, whether as a part of a carbon cementor as a custom carbon system. The resin per se may constitute up toabout 50 percent or more by weight of the coating composition. Althoughhigher resin concentrations are possible, little advantage is attained,and extended cure and carbonization cycles may be required. The carbonsystem should comprise about 10 to about 90 percent of the coatingcomposition, preferably from about 30 to about 80 percent, and mostpreferably from about 40 percent to about 65 percent of the coatingcomposition applied to the substrate.

One may utilize appropriate blends of carbon and phenolic resin or otherthermosetting resinous binders of the appropriate particle sizes, oralternative commercial compositions. The mix liquid component of thecoating composition may vary from approximately 2 weight percent toabout 40 weight percent for reasonable evaporation and curing rates,with about 5 percent to about 25 percent being preferred to obtainworkable consistency. It is desired that the coating composition beworkable and easily spread, as by a trowel. Insufficient liquid willmake the mix dry and unspreadable, while excessive liquid results indifficulties in curing and baking.

Various modifying agents may be present to modify the nature of theresinous binder during mixing, curing, and carbonization of the coatingcomposition. These may typically constitute from zero to about 10percent by weight of the coating composition. Suitable modifying agentsfor phenol formaldehyde resins, for example, include rosin, aniline,copolymers, resin "alloys", etc. Rosin modified phenolics have animportant combination of solubility, viscosity and dryingcharacteristics. A well known method of preparing phenolic resins foruse in surface coatings involves the heating and blending ofphenol-formaldehyde condensates with rosin. Preferably the condensateused is one made from alkyl phenols by alkaline condensation.

Aniline modified phenolics are prepared by treatment of an intermediatephenol formaldehyde condensate with aniline or aniline derivatives, byco-condensation, or by blending a phenol formaldehyde condensate with ananiline-formaldehyde condensate. Aniline modified phenolics haveparticularly good electrical characteristics. Phenolic resins may becopolymerized with such materials as chlorinated phenols, nitromethane,and organosilicon compounds, e.g. siloxane.

Some of the materials used to treat intermediate phenol formaldehydecondensates are epichlorohydrin, ether resin, ethylene oxide polymers,hydrogen peroxide, ketones, methylolaniline HCl, polyvalent salts ofhexanoic acid, stannous chloride, and terpene-phenol products.

In addition, blends of various resinous materials with phenolic resinsmay be prepared. Exemplary of the more common resin "alloys" arephenolic resins and epoxy resins, ketone-aldehyde condensates, melamineand urea resins, natural and synthetic rubber, polyvinyl chloride, andpolyvinal acetal resins.

Through such modifications, it is quite possible to considerably modifythe binder system. For example, replacement of phenol with meta-cresolwould yield a resin soluble in alcohol, having a fast drying time. Suchmight be advantageous under certain circumstances and cure timerequirements. Replacement of phenol with meta-alkyl phenol gives a resinwhich is more rubbery and flexible, but has less tensile strength.Replacement of phenol with substituted phenols, e.g. p-tertiary butylphenl, yields resins which are oil soluble, while replacement of phenlwith naphthalene and anthracene derived phenls does not alter thephenolic prperties greatly but could yield greater compatibility withpitch, in binder systems employing pitch. Replacement of formaldehydewith higher aldehydes, such as acetaldehyde, results in the resinbecoming oil soluble.

The addition of glycerol to phenol formaldehyde resoles acts as aplasticizer in the binder systems used in the present application.Anthracene oil, on the other hand, may be added to furfuryl alcohol as astabilizer prior to the addition of acid catalyst. Stabilization assistsin controlling the rate of polymerization in the presence of acidcatalysts.

Certain specific blends are noteworthy. For example, phenol formaldehydemodified with phenol-furfuryl resin has a flat plasticity curve, anddoes not go through a rubbery stage such as exhibited by phenolformaldehyde. Hence, blends of phenol formaldehyde with phenol-furfurylare suitable. They offer excellent impact resistance, chemicalresistance, rapid cure at high temperatures, and a high capacity forfiller. Blends of phenol formaldehyde with certain thermoplasticsproduce resins suitable for bonding to metal, such as may be consideredfor use with metallic cathode substrates. Blends withbutadiene/acrylonitrile copolymer rubber give improved impactresistance, while blends with resorcinolformaldehyde resins give fasterreaction rates and lower cure temperatures. These resins, however, aresomewhat more expensive for utilization in the present invention.

Frequently, pitch is present in the coating composition, as a modifyingagent or a binder, in concentrations of up to about 50 or even 75percent when present as a functional ingredient of the binder system.When present as a modifying agent per se, pitch may be present inconcentrations up to about 10 percent by weight of the ent in thecommercially available cements mentioned heretofore. Further particulatecarbon may be added, as either fine powder or coarse aggregate, ormixtures thereof, in the form of amorphous carbon or graphitic carbon.

It is highly desirable to have a carbonaceous filler material present,either as a component of a proprietary carbon system or present in acommercial cement, or as an addition to a commercial cement. Suchcarbonaceous filler is -100 mesh, and preferably -325 mesh, and maycomprise fine carbonaceous flour, graphite flour, crushed coke, crushedgraphite, carbon black, and the like. The presence of such fine floursyields improved packing density for the granulometry used, that wicks upresin forming liquids to form a dense, highly bonded carbon matrix uponcarbonization.

Carbonaceous filler, as fine flour, should comprise from about 1 percentto about 60 percent of the coating composition, with about 10 percent toabout 40 percent being preferred.

The carbonaceous additive, or aggregate material, if present, may runfrom -4 mesh to +100 mesh, and is preferably between -8 mesh and +20mesh. Such coarse aggregate apparently permits micro-cracking, assistsvolatile emission release, reduces shrinkage, and contributes to highcarbon yield. Carbonaceous additive, as aggregate and/or fiber, shouldcomprise from about 0 percent to about 70 percent of the coatingcomposition, with from about 5 percent to about 15 percent beingpreferred.

As previously set forth, it is preferred that carbon fiber be added tothe coating composition as a crack arrestor. When such fiber is used,some variations in composition ranges have been found. When carbonfibers are used, they may preferably be made from pitch precursors,organic fiber precursors such as polyacrylonitrile, or rayon. Pitchfibers are considerably cheaper, and accordingly preferred. Fiber weightmay range from zero percent to about 10 percent by weight of the coatingcomposition, preferably from about 0.05 to about 1.0 percent, and morepreferably from 0.05 to about 0.5 percent. However, concentrationsgreater than about 10 percent become comparatively expensive, withlittle apparent added benefit. Carbon fibers with lengths varying fromabout 0.16 cm to 1.27 cm length are preferred. Short fibers permiteasier mixing and application, and may be used in higher concentration.Sized fibers, consisting of parallel fiber strands bonded together by amaterial soluble in the mix liquid, are particularly preferred, sincethey blend most easily with the binder system. Fiber orientation mayvary, and the fibers can be mixed as an integral part of the coatingcomposition to facilitate the application procedure.

It is also possible to modify the carbonaceous filler utilized in thepresent invention. In this respect, a number of modifying agents may beadded to the filler. For example, silica may be added to thecarbonaceous filler to impart non-sintering properties to the bindersystem. In general, however, inorganic fillers make the resinous binderharder to process in accordance with the present invention and decreasechar yield.

Gas release agents are appropriately included in the coating compositionto avoid blisters and/or excessively large cracks. Suitable gas releaseagents include combustible oils, soaps, and waxes.

A combustible oil can be employed as the gas release agent in thecarbonaceous binder. In order to avoid volatilizing the oil while curingthe phenolic resin, it is desirable that the oil have a boiling pointhigher than the curing temperature of the resin. For this reason, oilshave a boiling point above about 150° C., preferably, above about 200°C., are most useful, with oils having a boiling point above about 250°C. being particularly preferred. While petroleum-base oils, such asparaffin oils, aromatic oils and asphaltic oils, are preferred, otheroils, such as animal and vegetable oils, can also be employed. Among thepetroleum-base oils, the paraffin oils are preferred. Illustrative ofthe animal and vegetable oils which can be employed are palm kernal oil,olive oil, peanut oil, beef tallow oil, cottonseed oil, corn oil,soybean oil, and the like. Amounts of oil of up to 3.5 percent by weightof the coating composition, preferably from about 0.4 percent by weightto about 2.0 percent by weight, are suitable.

Soaps may also be used as gas release agents. While the soap employedcan be any of the metallic or quaternary ammonium salts of the fattyacids, binders prepared with either the neutral or acid quaternaryammonium soaps are more resistant to oxidation than cements preparedwith the more common metallic soaps, so that the use of the non-metallicsoaps is preferred. Such non-metallic soap is produced by the reactionof a fatty acid with triethanolamine. The fatty acids employed, like thefatty acids employed to produce metallic soaps, generally contain fromabout 10 to about 24 carbon atoms, and can be either saturated orunsaturated. Among the saturated fatty acids which can be employed arecapric, lauric, myristic, palmitic, stearic, arachidic, behenic,tetracosanoic, and the like. Typical unsaturated fatty acids includepalmitoleic, oleic, linoleic, arachidonic, cetoleic, erucic,selacholeic, and the like. Amounts of soap of from about zero percent byweight to about 15 percent by weight or higher of the coatingcomposition, preferably from about 0.5 percent by weight to about 5.0percent by weight, are suitable.

Various waxes may also be employed as gas release agents. Suitable waxesinclude various grades of petroleum wax including such usual paraffinwaxes as refined slack, sweat, scale, block, and microcrystalline wax.

A preferred binder system is that which is commercially designated asUCAR® C-34, marketed by Union Carbide. This composition is believed tocomprise a mixture of an oil, a soap, finely-divided carbonaceousparticles, furfuryl alcohol, a phenolic resin of the novolac type, and ahardening agent for the phenolic resin. The mixture of the oil,finely-divided carbonaceous particles, phenolic resin, and phenolicresin hardener can be prepared by blending the carbonaceous particles,phenolic resin and phenolic resin hardener together in any conventionalmanner, e.g. in a tumbling barrel, spraying the oil into the resultingmixture, and further blending the mixture until the oil has beenincorporated therein and a substantially homogenous blend formed. Themixture of soap and furfuryl alcohol can be prepared by heating the soapup to a temperature of about 100° C. to liquify it, and then dissolvingthe molten soap in the furfuryl alcohol. Upon cooling, the soap remainsdissolved in the furfuryl alcohol as a stable solution which can bestored until it is ready to be mixed with the mixture of oil, finelydivided carbonaceous particles, phenolic resin, and phenolic resinhardener. The two mixtures, one liquid and the other essentially solid,can be readily mixed at room temperature, either manually ormechanically.

Many phenolic resins of the novolac type can be employed in the UCAR®C34 cement. Such resins are produced by the condensing phenols, such asphenol itself, m-cresol, p-cresol, o-cresol, 3,5-xylenol, 3,4-xylenol,2,5-xylenol, p-ethylphenol, p-tert-butylphenol, p-amyl-phenol,p-tert-octylphenol, p-phenylphenol, 2,3,5-trimethylphenol, resorcinol,and the like, with aldehydes such as formaldehyde, furfuraldehyde,acetaldehyde, and like. In practice, an unsubstitutedphenol-formaldehyde resin may be employed for cost considerations.Curing of the novolac resin to the thermoset state can be effected bymeans of any hardening agent conventionally employed to cure suchresins. Such hardening agents are conventionally materials such asparaformaldehyde, furfural, or hexamethylenetetramine, with appropriatecatalysts when necessary, which upon the application of heat generatealdehydes which react with the resin and cause it to crosslink. Thenovolac resin is suitably employed in the UCAR® C-34 cement in an amountof from about 0.5 percent by weight of the coating composition to about15 percent by weight, most preferably from about 2.5 percent by weightto about 8 percent by weight. The hardener for the resin is employed inan amount sufficient to cure such resin to the thermoset state, i.e., inan amount which will provide at least sufficient formaldehyde to reactwith and crosslink the resin, and provide sufficient alkaline catalystfor the reaction.

Many forms of finely-divided carbon or graphite can be employed ascomponents of the UCAR® C-34 carbonaceous cement. Suitable carbonaceousmaterials include graphite flour, petroleum coke flour, pitch cokeflour, calcined lampblack flour, thermatomic black (made by the passageof natural gas over hot refractories), and the like. Amounts of thecarbonaceous flour of from about 1 percent by weight of the coatingcomposition to about 60 percent by weight, preferably from about 10percent by weight to about 40 percent by weight, are suitable. Mostpreferably, the carbonaceous flour is a mixture of graphite andthermatomic black, with the graphite flour being present in an amount offrom about 2 percent by weight to about 50 percent by weight and thethermatomic blace being present in an amount of from about 0.5 percentby weight to about 25 percent by weight.

Suitably, furfuryl alcohol is employed in UCAR® C-34 cement, and may bepresent in an amount of from about 2 percent by weight of the coatingcomposition to about 40 percent by weight, most preferably from about 4percent by weight to about 20 percent by weight.

Additional suitable carbon cements are commercially available, such asUCAR® C-38, (Union Carbide), a composition very similar to UCAR® C-34but containing an oxidation inhibitor; Stebbins AR-25-HT, a furane resincomposition comprised of furfuryl alcohol, partially polymerizedfurfuryl alcohol in forms such as difurfuryl ether, and a latentcatalyst which could be phthalic anhydride or a derivative thereof;Stebbins AR-20-C, comprising a partially polymerized furane resin informs such as difurfuryl ether, together with furfuraldehyde, and alatent catalyst which could be phthalic anhydride or a derivativethereof; and Atlas CARBO-KOREZ, a phenolic resin composition comprisinga phenolic novolac or resole resin in a solvent as mix liquid, cured bycombination with a phenolic novolac in the solids. The solvent isprobably an aliphatic alcohol such as butyl alcohol. Other suitablecarbon elements include Atlas CARBO-ALKOR, comprising furfuryl alcoholmonomer and partially polymerized forms such as difurfuryl ether and alatent catalyst which could be phthalic anhydride or a derivativethereof; DYLON GC, comprising furfuryl alcohol as solvent and monomerwhich cures together with a phenol formaldehyde andhexamethylenetetramine hardener; and Aremco GRAPHI-BOND™ 551-R,comprising furfuryl alcohol and a latent catalyst.

The coating composition may be applied to the cathode of an aluminumcell as single or multiple layers. A multiple layer coating system mayprovide a stronger bond, due to greater penetration of the porestructure of the carbon cathode by a first bonding layer which does notincorporate TiB₂, and permits easier and more rapid cure of the coating.Further, the use of plural layers may also reduce the size and number ofshrinkage cracks in the TiB₂ -containing top layer. In still anotherpreferred embodiment, the coating composition may typically comprise upto about 10 percent by weight of carbon fiber, which acts to inhibitcrack formation, strengthen the coating, and lessen any tendency forexfoliation of the coating, particularly at any point of contact withthe bath. The carbon cement that is applied to the cathode substrate asa bonding layer may contain up to 40 percent extra carbonaceous fillerand additive, which help to prevent cracking of the substrate due tostress forces encountered during curing and carbonization of thecoating, by modifying the strength of the bond between the substrate andthe bonding layer, and the properties of the bonding layer.

The improved operation of full-scale (105K amp) VSS aluminum reductioncells constructed with coated cathodes in accordance with this inventioncontinues to demonstrate the productivity improvements resulting fromthis invention. While the examples below illustrate the preferredspecifications of the improved coating composition and process, it ispossible to obtain a viable coating by many similar procedures. Theexamples were prepared by coating the cathode blocks prior to theirassembly on the collector bars and the cell ramming process. Incommercial practice, it is possible that an in-cell coating would beused to coat all or part of the bottom and side wall surfaces of arammed cathode. The entire cathode coating could then be cured andcarbonized in a single operation using, for example, a hooded heaterplaced over the coated rammed cathode. This could result in economies intime and costing costs and provide a fully monolithic cathode withoutram joints, as well as reducing bake-in emissions and improving thebaking process to give an improved baked ram in the slots and sidewalls.

The coating composition can be applied to each cathode block, cured, andcarbonized before being set into position. Alternatively, the cathodecan be assembled and rammed, the coating applied and then cured. Thecarbonization process would occur in the cell in this case. Curing maybe accomplished in stages, whereby the coated substrate is graduallyraised to the desired curing temperature at which a relatively hardsurface is obtained, followed by carbonization at temperatures up to1100° C. In the initial states of such curing, volatile components, suchas the mix liquid volatiles and reaction products are removed, while athigher temperatures, e.g. 250° C. to 1100° C., carbonaceous materials,such as cross-linked phenolic resin, are decomposed to leave anon-graphitizing carbonaceous matrix containing RHM. This carbonizationstep may be carried out directly after the initial cure by heating thecoated carbon substrates to the desired temperature, or subsequent tosaid cathode substrate being placed in the electrolytic cell.Alternatively, the carbon cathode blocks may be placed in position inthe cell after coating and initial curing, followed by "bake-in" of thecoated cathode by cell start-up and operation. Alternative orders ofcoating, curing, carbonizing, assembling, setting, and ramming may ofcourse be utilized.

The area coated may range from the entire inner surface of the cathodecavity to less than 10 percent of the cathode surface below the anode oranodes. The area to be coated ranges from 50 to 100 percent of thecathode surface directly below the anode or anodes, with the preferredarea ranging from 70 percent to 100 percent of said area. It may,however, be desirable to leave some uncoated area, to permit cathode ramdegassing during cell heat-up and start-up, for example.

The RHM coating need not be continuous over the entire cathode surface.In the case of TiB₂ tiles, small gaps between adjacent tiles (1 mm to 5mm) will be bridged by the molten metal. Similarly, TiB₂ particles in acarbon surface at an appropriate concentration will produce apseudo-continuous aluminum wetting film by bridging between adjacentTiB₂ particles. In the case of the coating composition of thisinvention, 20 weight percent TiB₂ in the surface will produce apseudo-aluminum wetted surface. A preferred overall content of TiB₂ inthe surface layer of 35-60 weight percent will allow for mixinginhomogeneities and a longer coating life. Modification of the RHMparticle properties and/or changing the coating formulation and/or theRHM distribution within the coating may enable the use of lesser amountsof RHM. Cracks in the coating should be less than 5 mm in width,preferably less than 1 mm wide.

Two different types of TiB₂ powder have been used in the test cells. Nodifference could be detected in the mixing and coating procedures whenthe high purity (+99.5%) TiB₂ was replaced by less pure (98%) material.A carbothermic process was used to produce the high purity powder whilean arc melting process was used to produce the lower cost, less purepowder.

While no minimum or maximum coating thickness has yet been defined,thicker coatings provide longer coating life. However, the tendency isgreater for crack formation and higher coating costs with thickercoatings. The preferred coating thickness is from about 1 cm to about1.6 cm to minimize the tendency for blister formation or crack formationin the coating during cure and carbonization. Maximum coating thicknessshould be consistent with anticipated cell life; i.e. there is no needto have a coating thickness to last 10 years if cell life is anticipatedto be only 7 years.

It is also contemplated that the coating composition may be applied as asingle layer or as of a plurality of layers, which layers may beindividually cured between applications. In accordance with thisconcept, it is possible to produce a substantial coating thickness (e.g.5 cm or more) by successively applying thin layers of coatingcomposition and curing such layers individually. For greatest bondingstrength, it may be desirable to treat the surface of each cured layer,by scuffing or wire brushing, for example, prior to application of thenext layer. It is also possible, by this technique, to obtain agraduated RHM content within the coating, by changing the RHMconcentration in successive layers of coating composition.

The general procedure used to coat individual cathode blocks, assemblethe cathodes, and cut-in cells is summarized below.

1. Wire brush and vacuum clean the top surface of the cathode blocks.

2. Attach mold around the top of each block.

3. Mix dry coating components.

4. Preheat coating materials and blocks.

5. Mix the coating composition.

6. Apply a thin coating and work it into the block surface.

7. Complete application of coating and use a bar to level coating withtop of mold.

8. Wait 50-30 minutes before using a metal trowel to semi-smooth thecoating surface.

9. Insert a thermocouple in the side of the coating and place coatedblocks in cure oven.

10. Cure.

11. Remove coating molds from the blocks.

12. Place the cured blocks in a metal box, cover with coke and heat topartially carbonize.

13. Cool the blocks before removing them from the coke bed.

14. Cast the blocks on collector bars.

15. Ram the collector bar assemblies in cathode shell and ram sidewalls.

16. Wire brush excess ram off the coated block surface.

17. Follow conventional procedures for cathode installation, bake andcut-in.

The uniformity, workability and bonding properties of the coating arestrongly influenced by temperature. The preferred premixing temperaturesof the mix liquid and solid portions of the coating composition are40°-45° C. and 30°-35° C., respectively. A premixing temperature rangeof 20°-45° C. has been used for the mix liquid and solid componentswithout any severe problems. The temperature of the cathode block orsurface to be coated should be between 20° C. and 65° C., preferablybetween 40° C. and 50° C.

A low premixing temperature resulted in non-uniform mixing and the needto add more solvent to make the coating workable. Both produced smallblisters in the coating. A high premixing temperature caused a partialpremature curing of the coating which resulted in poor workability andpoor bonding to the carbon substrate.

A low block temperature made it difficult to uniformly spread thecoating and achieve the required wetting of the carbon substratenecessary for good bonding. A high block temperature caused partialpremature curing and excessive shrinkage in the coating. Excessiveshrinkage increased the tendency for crack formation and delaminationfrom the carbon substrate.

As indicated, certain ranges of acceptable temperatures for preheattreatment of the coating composition and the cathode blocks exist. It isalso noted that coating thickness may vary from approximately 0.6 cm to1.6 cm or higher. Preferred ranges are set forth in Table I.

                  TABLE I                                                         ______________________________________                                        Application Parameters                                                                         Ranges                                                       Item               Acceptable                                                                              Preferred                                        ______________________________________                                        Preheat dry components                                                                           20-45° C.                                                                        30-35° C.                                 prior to mixing                                                               Preheat liquid component                                                                         20-45° C.                                                                        40-45° C.                                 prior to mixing                                                               Preheat cathode blocks                                                                           20-65° C.                                                                        40-50° C.                                 prior to coating                                                              Coating thickness  0.6-1.6 cm                                                                              1.0-1.3 cm                                       ______________________________________                                    

The tendency for blister formation in the coating was affected by thedegree and technique used to finish the top surface of the applied wetcoating. A finished smooth surface achieved by either dry or wet workingthe coating surface exhibited a greater tendency to form blisters than arough trowelled surface. A smooth surface promoted the rapid formationof a film, which sealed the surface and interfered with the release ofblister forming gases evolved during curing. Conversely, theimperfections in the partially smoothed surface promoted the release ofgas during the cure cycle. The surface of eight blocks coated with theformation given in Example 6 were finished smooth by both wet and dryworking, and blisters were observed in the coatings of these eightblocks, after curing. Blister-free coatings on 52 subsequent blocksusing the same formulation were achieved by only partially smoothing thesurface of the wet coating.

The surface texture of different finishes on coatings prepared from thecoating composition of Example 8 has also been characterized. Thetechnique in this instance was to take a 2 cm×2 cm×2 cm sample of coatedblock and mount this in epoxy resin filled with white powder to achievea white background against the black coating. The mounted specimen wasthen sectioned and polished to reveal the detailed profile of thecoating surface. This was photographed at 12X magnification and theoutline of the coating surface traced onto plain paper. "Typical" 5mmsections of this profile were then analyzed in terms of a maximumpeak-to-valley height and the average calculated. Values of thesemeasurement are set forth in Table II.

                  TABLE II                                                        ______________________________________                                        Coating Roughness for Coating CM-82                                                         Average Peak-to-Valley Height                                                 Taken on Typical 5 mm Lengths                                   Surface Finish                                                                              of Coating Surface                                              ______________________________________                                        unfinished    1.25 mm                                                         semi-smoothed 0.74 mm                                                         fully-dry smoothed                                                                          0.62 mm                                                         fully-wet smoothed                                                                          0.26 mm                                                         ______________________________________                                    

It was noted that coated cathodes which were given a fully-dry orfully-wet smooth finish exhibited blistering, whereas those given to asemi-smooth finish resulted in acceptable coating. Accordingly, it maybe speculated that a surface texture having average peak-to-valleyheights of less than about 0.65 mm is to be avoided.

The high porosity of graphite particles (UCAR® BB-6) or other equivalentporous aggregate enhanced the escape of solvent and other gases duringthe cure and carbonization cycle, compared to that of regular cokematerial such as UCAR® 6-03 coke. Addition of a porous aggregate to thecoating reduced the tendency to form blisters and the amount of coatingshrinkage during curing. Excessive shrinkage weakens thecoating-substrate bond strength and enhances crack formation.

EXAMPLE 1: CHAR YIELD

Accurately known weights of mix liquid and solid components of variouscarbon systems were mixed in a Teflon® coated mold having dimensions 5.1cm×1.27 cm×0.63 cm deep. When the system comprised a separate hardener,the weight of hardener was included with the mix liquid weight. Eachcarbon system was taken through the usual cure cycle, then weighed andremoved from the mold. The cured piece was then heated to constantweight in an alumina crucible at 250° C. in air, and the weightcorrected for any loss which might have occurred in removing the piecefrom the mold (e.g., breakage, dusting or adhesion to the mold). Theextrapolated weight was thus the "true" weight that the sample wouldhave attained at 250° C. in the absence of physical manipulation. Thepiece was then sintered to 1000° C. over a period of 24 hours in analumina crucible under an Argon atmosphere. After furnace cooling, thesample was weighed, and the weight again corrected to correspond withthe original weight of composition (rather than the weight used in thesintering phase). The adjusted char weight thus represents the totalchar weight resulting from the original sample.

An accurately determined weight of the original solids was extractedwith boiling methyl ethyl ketone (MEK) in a soxhlet extraction apparatusfor 4 hours, or longer if the extract continued to emerge colored afterthat time. The weight of extracted material was determined byevaporating the pregnant solvent to dryness (at least 24 hours) in atall glass beaker held in a vacuum oven at ambient temperature. Theweight increase over that of the empty beaker was taken as the weight ofextracted material. This was cross-checked with the weight of insolublesolids remaining in the soxhlet extraction thimble (similarly evaporatedto dryness and constant weight under vacuum).

The weight of MEK insolubles in the original carbon system is thencomputed from the known weight of solids originally present and thepercentage of those solids which comprise the MEK insolubles.

These solids were assumed not to undergo any weight loss to 250° C.Thus, the weight of "resin" (in this case defined as everything apartfrom MEK insolubles in the carbon system cured to 250° C.) in the pieceat 250° C. is given by the "true" piece weight at 250° C. less theweight of MEK insolubles originally present.

The char weight due to the MEK insolubles was determined by sinteringthe extraction residue from the soxhlet thimble to 1000° C. under anArgon atmosphere in an alumina crucible. The resultant char weight wasagain adjusted to correspond with the actual weight by subtracting MEKinsolubles found in the sample initially.

The char weight from the resin was thus the total char weight minus thechar weight from MEK insolubles. And the "char yield" was the charweight from the resin as a percentage of the total resin weight in theslug at 250° C.

Results from these measurements are as follows:

                  TABLE III                                                       ______________________________________                                        Char Yield                                                                                     Char Yield                                                   Carbon System    of Resin  Coating Quality                                    ______________________________________                                        UCAR ® C-34 cement                                                                         71%       Excellent-high                                                                density                                            Atlas CARBO-VITROPLAST                                                                          8%       Poor-Brittle,                                                                 porous, spongy                                     Stebbins AR-20-C Cement                                                                        59%       Good-high density                                  Coating composition CM-82                                                                      94%       Excellent-high                                     (Example 8       density                                                      ______________________________________                                    

EXAMPLE 2 EXPANSION VALUES

A sample of a coating composition was prepared and spread in a Teflon®coated mold having dimensions 5.1 cm×1.25 cm×0.63 cm deep. This wastaken through the usual cure cycle, allowed to cool, and the sampleremoved from the mold. The sample was cut into four test pieces ofdimensions 2.5 cm×0.6 cm×0.6 cm which were dried to constant weight inan alumina crucible at 250° C. (usually 16-24 hours).

A test-piece was measured using a micrometer, then heated from roomtemperature to 1000° C. in a dilatometer which was continuously flushedwith Argon. The dilation/contraction was recorded continuously as afunction of temperature on an XY recorder. Two shrinkage values werecalculated: one based on the original sample length together with thelength at 1000° C.; the other based on the original length together withthe final length once the sample had cooled back to room temperature.Final lengths of the samples were also measured with a micrometer aftercooling as a double check on the recording system. The samples testedwere as set forth in Table IV.

While the overall expansion of the coating materials appears to showrelatively poor reproducibility, in spite of very careful samplepreparation and uniform temperature cycling, it is noted that aftercarbonization, expansion of the coating material between 800° C. and1000° C. shows good reproducibility, and is very similar to blockexpansion. It is further noted that the UCAR® C-34 cement by itselfdisplays an unacceptable expansion over this temperature range.

It is believed that up to the point of full carbonization (about 800°C.), the coating has sufficient yield to accomodate any mismatch inC.T.E. between coating and cathode substrate. Beyond this point thecoating becomes rigid and brittle, so that if a difference in C.T.E.exists, it may only be accomodated by development of internal stresses,or by opening up of larger cracks.

                                      TABLE IV                                    __________________________________________________________________________    Expansion Values                                                                         Overall          Overall                                                      Expansion                                                                             Expansion*                                                                             Contraction**                                     Material   20° C.-1000° C.                                                         800° C.-1000° C.                                                         20° C.-1000° C.-20°          __________________________________________________________________________                                C.                                                Cathode Block                                                                 Sumitomo S.K.                                                                            0.49%   0.12%    0                                                 UCAR ® CFN                                                                           0.33%   0.08%    0                                                 Stein      0.48%   0.12%    0                                                 Savoie HC-10                                                                             0.36%   0.08%    0                                                 CM-78B #1  0.12%   0.15%    -0.52%                                            #2         0.01%   0.11%    -0.52%                                            CM-82 #1   0.32%   0.08%    -0.79%                                            #2         0.13%   0.10%    -0.63%                                            UCAR ® C-34                                                                          1.8%    -0.14%   -2.3%                                             Cement                                                                        (No additive, no fiber)                                                       __________________________________________________________________________     *Curves from the dilatometer show that carbonization is complete at about     800° C. Therefore the expansion between 800° C. and             1000° C. represents the expansion of a coating which has lost          essentially all its plasticity.                                               **The overall contraction is based on the original length of the sample,      and the final length once it has been through the temperature cycle and       returned to room temperature.                                            

EXAMPLE 3

A coating composition (CM-24A) was made by combining and mixing thefollowing components (percentages are by weight): 36 percent TiB₂, -325mesh; 34.2 percent Union Carbide UCAR® C-34 carbon cement solids, 19.7percent UCAR® C-34 mix liquid; 10.1 percent Union Carbide calcinedpetroleum coke particles, grade 6-03.

The resulting coating composition was applied to a 7.5 cm×15 cm cathodeblock substrate. Enough material to make a layer approximately 0.16 cmdeep was applied and worked into the block surface. Additional materialto make a layer approximately 1.59 cm thick was added, smoothed andlevelled.

The coating was cured by heating at 25° C./hour to 100° C., holding 5hours, heating at 25° C./hour to 140° C., holding 24 hours, and aircooling to room temperature.

After curing, several small cracks were evident but thecoating-to-substrate bond was intact.

EXAMPLE 4

A coating composition (CM-37) was made by combining and mixing thefollowing components: 36 percent TiB₂, -325 mesh; 29.4 percent UCAR®C-34 cement solids; 32.4 percent UCAR® C-34 mix liquid; 2.2 percentGreat Lakes Carbon FORTAFIL®0.63 cm length unsized fiber.

The resulting coating composition was applied to a 7.5 cm×15 cm cathodeblock substrate and cured in a manner similar to that described inExample 3.

After curing, the coating exhibited no cracks such as were evident inthe coating with the carbon aggregate used in Example 3. The carbonfibers appear to act as crack arrestors in the cathode coating, and soshould result in a longer cathode life.

EXAMPLE 5

A 7.5 cm×15 cm piece of cathode block was utilized as a substrate for acoating composition(CM-38A) consisting of 37.5 percent TiB₂, 30.6percent UCAR® C-34 cement solids, 29.6 percent UCAR® C-34 mix liquid,and 2.3 percent FORTAFIL® unsized carbon fiber. This material wasapplied in a manner similar to that of Example 3, to a final thicknessof 0.95 cm.

The coated substrate was then cured in accordance with the followingcycle: The coated block was heated to 100° C. at a rate of 25° C. perhour, and held at this temperature for 3 hours. Heating was continued at25° C. per hour to 140° C., at which temperature the block was held for16 hours. The coated block was then removed and allowed to air cool toroom temperature. No surface blisters, cracks, or bond defects werevisible upon inspection.

The cured block was then carbonized by heating to 1000° C. over a 24hour period, in an argon atmosphere to avoid oxidation. The block wasthen permitted to cool to below 200° C. under an Argon atmosphere,removed and cooled to room temperature. After cooling, examinationrevealed no defects. Integrity and substrate bond remained unaffected bycarbonization.

EXAMPLE 6

The mix liquid content was reduced from about 20-32 weight percent toabout 15-19 weight percent, in order to scale up the coating process forcoating small test blocks (7.5 cm×15 cm) to that for full-scale cathodeblocks (50.2 cm×52.1 cm). Blisters formed during the cure cycle whenhigher solvent formulations were used to coat the larger cathode blocks.As the surface area to circumference ratio for the block to be coatedincreased, there was an increasing tendency for the coating to formblisters, i.e., the solvent could not escape during the cure cycle.Blistering also tended to increase with coating thickness, i.e., 0.32 cmthick coatings had little tendency to blister whereas 1.27 cm thickcoatings had a large tendency to blister. Blister-free coatings wereobtained on cathode blocks using the following formulation (CM-66):

36 weight percent TiB₂ powder, -325 mesh

36.35 weight percent UCAR® C-34 cement solids

19.3 weight percent UCAR® C-34 mix liquid

0.35 weight percent Great Lakes Carbon FORTAFIL® 3, unsized carbonfibers, 0.63 cm length

8.0 weight percent UCAR® BB-6 Graphite particles

A commercial VSS aluminum reduction cell was operated experimentally for310 days using a cathode coated 0.63 cm thick with the aboveformulation. The cell energy efficiency was 0.14 kwh per pound ofaluminum produced lower than the plant average for the last nine monthsof operation.

After 310 days of operation, the test cell (A-43) was shut down and themolten metal and bath tapped so that less than 5 cm of metal remained inthe bottom of the cathode cavity. Examination of a 4.5 cm diameter coresample from the cold test cathode revealed that the coating loss wasconsistent with the projected coating life as determined from metalanalysis (i.e., approximately 1/3 of the original coating thickness hadbeen lost). In isolated areas of the cathode, there was evidence thatthe entire coating had been lost.

Core samples from test cell A-46 in Table V, which was also shut down,confirmed that the coating loss was consistent with the projectedcoating life as determined from metal analysis. There was no evidence ofisolated areas of coating failure or muck formation on the cathodesurface of this cell.

EXAMPLE 7

The tendency to form blisters was reduced by decreasing the fibercontent in the coating, because reducing the fiber content decreased theamount of solvent required to make the coating workable. However, therewas a tendency for the formation of fine vertical cracks in the coating.These fine vertical cracks provided a means for the solvent to escapeduring the cure cycle, and hence, reduced blistering. Blister-free, butfinely cracked, coatings were obtained on cathode blocks using thefollowing formulation (CM-78B):

46.0 weight percent TiB₂ powder, -325 mesh

28.5 weight percent UCAR® C-34 cement solids

15.5 weight percent UCAR® C-34 mix liquid

10.0 weight percent UCAR® BB-6 Graphite particles

Many problems were encountered during the initial stage of this coatingexperiment due to an attempt to create a very smooth coating surface.After application of the coating composition to the cathode substrate,it was wet with excess mix liquid and completely smoothed. Such smoothcoatings developed severe blisters upon curing. The extra mix liquidapparently formed a "skin" on the surface during cure, and interferedwith the escape of evolved gases. This resulted in blister formation anddefective coatings. When the surfaces of subsequent coatings were givena light surface finish without excess liquid, no blister formationoccurred, and acceptable cured surfaces resulted. It was also noted thatambient temperatures had an effect upon the mixing and spreadability ofthe coating composition, with colder temperatures resulting in astiffer, more difficult coating application.

As long as vertical cracks remain small, they do not interfere with thefunction of the coating. The TiB₂ content was increased in thisformulation to increase the coating life, based on an assumed constantTiB₂ dissolution rate. A commercial VSS aluminum reduction cell wasoperated experimentally for more than 225 days using a cathode coated0.95 cm thick with the above formulation. The cell energy efficiency was0.19 kwh per pound of aluminum produced lower than the plant average,for the last six months of operation.

EXAMPLE 8

The formulation of Example 7 was modified to reduce the number ofvertical cracks in the coating while still maintaining a low solventcontent to prevent the formulation of blisters. Preferred formulations(CM-82 and CM-82A), the cured resinous binder of which has a char yieldof 94 percent, as determined by char yield analysis of this coating,follow:

CM-82

46.0 weight percent TiB₂ powder, Carborundum -325 mesh

28.0 weight percent UCAR® C-34 cement solids

15.8 weight percent UCAR® C-34 mix liquid

0.2 weight percent Great Lakes Carbon FORTAFIL® 3, carbon fibers sizedfor UCAR® C-34 cement, 0.32 cm length

10.0 weight percent UCAR® BB-6 Graphite particles.

CM-82S

46.0 weight percent TiB₂ powder, Metallurg -325 mesh

28.0 weight percent UCAR® C-34 cement solids

15.8 weight percent UCAR® C-34 mix liquid

0.2 weight percent Great Lakes Carbon FORTAFIL® 3, carbon fibers sizedfor UCAR® C-34 cement, 0.32 cm length

10.0 weight percent UCAR® BB-6 Graphite particles.

Cathode blocks for two commercial size VSS aluminum reduction test cellswere successfully coated to a thickness of 0.95 cm with the aboveformulations and operated for 125 days. The number of vertical cracks inthe coatings was reduced to virtually zero. The cell energy efficiencieswere 0.08 and 0.05 kwh per pound of aluminum produced lower than theplant average respectively, for the last two months of operation.

In preparing the coated cathode blocks of this Example, the blocks werepreheated to 40° to 50° C. prior to coating, due to cooler ambienttemperature, to give good coating composition workability without theneed to add additional mix liquid. It was also noted that preheating thedry and liquid components of the coating composition prior to mixingimproved the blending and application properties of the coatingcomposition. Further, at lower mix liquid concentrations, less blisterformation occurred.

Additional experiments were conducted utilizing various coatingcompositions and preheating the cathode blocks. It was found thatpreheating the block to 70° to 80° C. resulted in a very smooth and easycoating application, but caused premature curing, resulting in excessiveshrinkage and debonding. It was found that pre-heating temperatures of40°-50° C. resulted in the best combination of coating compositionworkability and elimination of shrinkage effects.

The foregoing coating formulations, and additional formulations preparedand utilized for commercial VSS aluminum cell cathode blocks aresummarized in the following Table V, wherein energy efficiency isexpressed as kwh per pound of aluminum produced, and projected coatinglife is based upon dissolution of TiB₂ into the aluminum product. At anenergy cost of 22 mils per kwh, savings on the order of $10 per ton areprojected.

                                      TABLE V                                     __________________________________________________________________________    Experimental Results                                                          __________________________________________________________________________    Composition (Weight Percent)                                                                 CM-66  CM-78B CM-84A CM-78B CM-82  CM-82S                      Cell           A-43   D-29   D-60   D-37   A-50   A-46                        Example        Example 6                                                                            Example 7            Example 8                                                                            Example 8                   TiB.sub.2      36.0   46.0   36.0   46.0   46.0   46.0                        UCAR ® cement solids                                                                     36.35  28.5   37.65  28.5   28.0   28.0                        UCAR ® mix liquid                                                                        19.3   15.5   18.0   15.5   15.8   15.8                        Aggregate       8.0   10.0    8.0   10.0   10.0   10.0                        Carbon fiber    0.35  --      0.35  --     0.20   0.20                        Days in Service                                                                              310    225    181    168    125    109                         Coating Thickness                                                                            0.63 cm                                                                              0.95 cm                                                                              0.95 cm                                                                              0.95 cm                                                                              0.95 cm                                                                              0.95 cm                     Projected life (days)                                                                        875    2000   2000   2000   2000   2000                        Energy Saving (kwh/#)                                                                        0.14   0.19   0.22   0.39   0.08   0.05                        TiB.sub.2 specification                                                                      Carborundum                                                                          Carborundum                                                                          Carborundum                                                                          Carborundum                                                                          Carborundum                                                                          Metallurg                   (-325 mesh)    99.5 percent                                                                         99.5 percent                                                                         99.5 percent                                                                         99.5 percent                                                                         99.5 percent                                                                         98 percent                  Fiber specification                                                                          0.63 cm       0.32 cm       0.32 cm                                                                              0.32 cm                     (Great Lakes Carbon                                                                          unsized       sized         sized  sized                       FORTAFIL ® 3)                                                             __________________________________________________________________________

In addition to the commercially available carbon cement UCAR® C-34,utilized in foregoing examples, a large number of other thermosettingcarbon cements have been utilized for the preparation of coatingcompositions. In most cases, successful coatings were prepared. In mostinstances where poor quality coatings resulted after curing andcarbonization, it is believed that successful coatings may be preparedby making specific modifications to the coating formulation.

EXAMPLE 9

A coating composition (CM-89) was prepared by combining and mixing thefollowing components: 36.5 percent TiB₂, 34.6 percent UCAR® C-37 carboncement solids, 19.2 percent UCAR® C-37 mix liquid, and 9.7 percent UCAR®BB-6 graphite aggregate. This composition is essentially identical tothe composition of UCAR® C-34 carbon cement, with the addition of acidwashed graphite flake carbonaceous filler for expansion. The coatingcomposition was relatively workable, and fluid, and was easily spreadwith little tool sticking upon a Sumitomo SK cathode block, 7.5 cm×15cm×5 cm, to a thickness of 0.95 cm.

This coated block was cured by elevating the temperature from roomtemperature to 100° C. in a 1-hour period. Holding 4 hours, heating to135° C. in 1 hour, holding at 135° C. for 4 hours, heating to 150° C. in1 hour, holding at 150° C. for 12 hours, heating to 165° C. in 1 hour,and holding for 3 hours at 165° C., followed by cooling in air. Thecured coating exhibited debonding due to shrinkage at the outside edgesof the substrate surface, with minor internal porosity, very minorlateral cracks, and no surface defects.

The coated substrate was then carbonized by heating to 1000° C. in anargon atmosphere over a 24-hour period, and allowed to cool to roomtemperature. The coating was essentially destroyed by the carbonization,with virtually no coating remaining on the substrate surface. Thisexample is illustrative of the importance of choice of the carbonaceousadditive utilized in the coating composition. In this specific instance,the coating composition included an incompatible additive in the carboncement resin, specifically a graphite flake additive suitable forcausing expansion.

EXAMPLE 10

A coating composition (CM-90) was prepared from a mixture of 35 percentTiB₂, -325 mesh, 33.4 percent UCAR® C-38 carbon cement solids, 22.2percent UCAR® C-38 mix liquid, and 9.4 percent UCAR® BB-6 graphiteparticles. The UCAR® C-38 cement is believed to be essentially identicalto the UCAR® C-34 carbon cement with the addition of an oxidationinhibitor. The coating composition prepared exhibited good workability,fluidity, and was easily spread onto a Sumitomo SK block as set forth inthe previous example. This coated block was then cured, as in Example 9.The cured coating composition illustrated very large sub-surfaceblisters, good bonding, good density, and no surface defects.

After carbonization as set forth in Example 9, the carbonized coatingexhibited debonding due to shrinkage of the coating composition, with nosurface defects appearing in the carbonized coating. It is believed thatthis coating composition could well be utilized in the present inventionby providing a bonding layer of carbon cement between the coatingcomposition and the cathode substrate.

EXAMPLE 11

A coating composition (CM-87) consisting of 33.6 percent TiB₂, 37.5percent Stebbins AR-20-C carbon cement solids, and 28.9 percent StebbinsAR-20-QC mix liquid was prepared. The Stebbins AR-20-QC mix liquid isbelieved to comprise a partially polymerized furane resin, such as afurfuryl alcohol low resin polymer. Furfural and a latent catalyst, suchas phthalic anhydride, are also believed to be present. The cementsolids of this carbon cement are believed to comprise a carbon powderwith catalyst, and an aniline modifier.

This coating composition was applied to a cathode substrate, aspreviously set forth, and cured at 30° C. for 24 hours, heated to 110°C. in 3 hours, and held for 24 hours at 110° C. The coating composition,which exhibited poor workability, and was stiff and sticky prior toapplication to the substrate, exhibited very minor lateral crackingafter curing. The cured coating was hard with no blisters or largecracks, has good density, and a very strong bond to the substrate. Aftercarbonization as set forth in Example 9, the carbonized coatingexhibited slight shrinkage cracking, slight debonding due to saidshrinkage, good density, but had large spalled areas.

EXAMPLE 12

A similar coating composition (CM-88) was prepared utilizing 38.7percent Stebbins AR-25-HT carbon cement solids, 28.3 percent StebbinsAR-25-HT mix liquid, and 33 percent titanium diboride. This cement isbelieved to comprise a modified furane resin, of furfuryl alcohol and alow resin polymer of furfuryl alcohol and a latent catalyst. The coatingcomposition was stiff and sticky, exhibiting poor workability. Thecoated substrate was cured as set forth in Example 11, resulting in acoated substrate exhibiting the same characteristics as those of thecoating of Example 11. After carbonization, the coating exhibited moreshrinkage than that of Example 11, causing debonding. No spalling orcracks were noted in the surface layer, however.

EXAMPLE 13

A polyester resin with carbon filler, marketed as Atlas CARBOVITROPLASTcarbon cement, was mixed with 35 percent TiB₂ to form a coatingcomposition (CM-92). The coating composition exhibited good workability,good fluidity, was not sticky, and was easily spread upon a cathodesubstrate as set forth in Example 9.

The coated substrate was cured by heating to 100° C. in 1 hour, holdingat 100° C. for 3 hours, heating to 165° C. in 1 hour, holding at 165° C.for 10 hours, heating to 200° C. in 1 hour, holding for 1/2 hours, andair cooling. The cured coated exhibited excellent bonding to thesubstrate, excellent density, and no surface defects. The coatedsubstrate was then subjected to carbonization, as set forth in Example9. The carbonized coated exhibited complete debonding, with the coatingcomplete but having extensive fine porosity. The coating displayed nosurface defects, but was very light and powdered easily. It is notedthat the resinous binder utilized in the preparation of this coatingcomposition exhibited a char yield of only 8 percent, illustrating thecriticality of this parameter.

EXAMPLE 14

Atlas CARBO-ALKOR, a carbon cement based upon furane resin with carbonfiller, was prepared in accordance with manufacturer's instructions andblended with sufficient titanium diboride to comprise 35 percent of thecoating composition. This coating composition (CM-93) exhibited verygood workability. After curing as set forth in Example 13, the coatingexhibited good bonding through the substrate, extensive surfacespalling, and reasonable density. After carbonization, the coatingexhibited slight debonding, more extensive spalling, and large cracks inthe surface due to shrinkage. When the coating composition wasreformulated by the substitution of 3.5 percent soap as gas releaseagent in place of an equal amount of mix liquid, an acceptable coatingwas obtained.

EXAMPLE 15

A coating composition (CM-94) was prepared as set forth in Example 13,utilizing Atlas CARBO-KOREZ carbon cement, which is believed to be aphenolic resin based cement with carbon filler, having a butyl alcoholsolvent. The coating composition exhibited very poor workability, beingextremely stiff and hard to spread on the carbon substrate. After curingas set forth in Example 13, the coating composition exhibited goodbonding, good density and no surface defects. After carbonization,however, debonding due to shrinkage was observed. It is believed thatthe substitution of a furfuryl alcohol solvent for the butyl alcohol mixliquid of the Atlas CARBO-KOREZ resin would result in improvedimpregnation of the cathode substrate, and hence better bonding.

EXAMPLE 16

A coating composition was prepared utilizing 35 percent TiB₂ and 65percent premixed DYLON grade GC carbon cement. The DYLON carbon cementis believed to comprise less than 10 percent powdered coal tar pitch,less than 20 percent phenolic resin, and less than 35 percent furfurylalcohol. The coating composition exhibited very good workability, butafter curing as set forth in Example 9, extensive large cracking of thecoating and debonding were observed. Substantial lateral cracking andhigh porosity were noted. This coating was not considered worthcarbonization in light of its poor condition after curing. This coatingcomposition was later reformulated with the addition of 3.5 percent soapas a gas release agent, and produced an excellent coating after cure andcarbonization. This illustrates the criticality of choice of additionalmaterials for the coating composition, and the failure of a coatingcomposition due to lack of porosity by which volatiles could escape.

EXAMPLE 17

A coating composition was prepared utilizing 45 percent TiB₂ and 55percent pre-mixed Aremco GRAPHI-BOND™ Grade 551-R carbon cement. Thiscarbon cement is believed to comprise 60 percent graphite, and 40percent furfuryl alcohol, and is specifically formulated for use inreducing atmospheres. The coating composition was smooth and relativelyeasy to apply to the cathode substrate. The coating substrate was thencured as set forth in Example 9, resulting in good bonding, reasonabledensity, with vertical cracking near the edges and extensive finehorizontal cracking. After carbonization, debonding occurred near theedges due to shrinkage, but the coating exhibited reasonable density,with some large vertical cracking and some fine horizontal cracking. Itis believed that the addition of gas release agent and carbonaceousadditive would overcome such problems.

EXAMPLE 18

A coating composition as set in Example 17 was prepared, utilizingAremco GRAPHI-BOND™ Grade 551-R, which is believed to comprise 32percent graphite, and 68 percent alumino-sodium silicate. The coatingcomposition was very fluid, and poured rather than spread. The coatedsubstrate was cured as in Example 9, but a huge blister developed withinthe coating after 4 hours at 100° C. A good bond was formed, althoughthe coating was extremely porous. After carbonization, the coatingappeared visably unchanged, but it had become more brittle and crumbly,and slight debonding was evident near the edges. This exampleillustrates a coating composition which is unsuitable in the presentinvention.

EXAMPLE 19

An additional coating composition was prepared utilizing 45 percent TiB₂and premixed Great Lakes Carbon Grade P-514 carbon cement, believed tobe based upon graphite particles and a binder. The coating compositionwas easily workable, and easily smoothed on to a cathode substrate.After curing, as set forth in Example 9, extensive large scale porosityand spalling occurred after 4 hours at 100 ° C. A good bond was formed,with good density in regions between holes. The coating was shiny, witha glassy appearance within pores and under the spalled areas. Aftercarbonization, the coated substrate appeared unchanged, except thatlarge vertical cracks had developed and slight debonding occurred alongthe edges. When the coating composition was reformulated by thesubstitution of 3.5 percent soap as gas release agent in place of anequal amount of mix liquid, an acceptable coating was obtained.

EXAMPLE 20

An intractable (infusible) polyphenylene polymer is prepared bypolymerization of 1,3 - cyclohexadiene with Ziegler-Natta catalystsfollowed by halogenation and dehydrohalogenation. This produces apara-phenylene polymer with a molecular weight of about 4,000. Thisresin alone is unsuitable for use in the present invention because ofthe relatively low molecular weight and relative insolubility.

A fusible polyphenylene resin (soluble, for example, intrichlorobenzene) is prepared by cationic oxidative polymerization of1,3,5-triphenylbenzene. The reaction product contains intractablepolyphenylene by-product which is removed by fractional distillation.The distilled fusible polyphenylene has a molecular weight between 1000and 1500.

A coating composition is prepared utilizing these two resins, asfollows. The two resins are ground separately to -325 mesh and thenmixed thoroughly for 15 minutes in a high speed blender, in the ratio1.6 percent (by weight of the coating composition) intractablepolyphenylene to 3.3 percent fusible polyphenylene. To this mixture isthen added 16.3 percent graphite flour (UCAR® GP 38 P) and 8.2 percentREGAL® 400 pellets (carbon black from Cabot Corporation).

A mix liquid is prepared containing 1.6 percent of triethanol amine(TEA) olate and 11.4 percent of chloroform. The TEA oleate is firstheated to a liquid state by warming to 100° C. and then poured into thechloroform.

To the dry components (comprising the polyphenylene resin powders,graphite powder and carbon black) are now added 47.3 percent Carborundum-325 TiB₂, 9.8 percent Union Carbide UCAR® BB-6 graphite aggregate and0.5 percent Great Lakes Carbon FORTAFIL® 0.32 cm sized fiber. Theingredients are dry blended and the mix liquid added until a uniformdispersion of the ingredients is observed.

The coating composition is spread onto a cathode block in the usualmanner to form a 0.95 cm thick layer. This block is then left in air for1 hour to allow some of the solvent to evaporate. Curing is achieved byslowly heating the coated block to 70° C. to volatilize most of the mixliquid, followed by slow heat to 80° C. and holding at this temperaturefor 2 hours. Again the temperature is slowly raised to 200° C. and heldfor 8 hours. The block is then cooled to room temperature.

A partial carbonization is then effected by heating for 9 hours at 135°C., followed by 24 hours of slow temperature elevation to 300° C. andholding at 300° C. for 6 hours. A satisfactory coating results.

EXAMPLE 21

A coating composition (CM-91) consisting of 44.9 percent TiB₂ (-325mesh, Carborundum), 28 percent UCAR® C-34 carbon cement solids, 14.7percent UCAR® C-34 mix liquid, 12 percent UCAR® BB-6 graphite, and 0.4percent Great Lakes Carbon FORTAFIL® 3 0.32 cm sized fiber was prepared.

This composition was difficult to mix due to high fiber content. TiB₂dispersion was poor, and the composition needed remixing half waythrough the coating operation.

This material was coated on a full size cathode block, preheatedovernight to 50° C. The composition was difficult to spread until heatedby the warm block.

The coating was levelled to a 1.27 cm thickness, semi-smoothed, andcured in the manner set forth in Example 9. No defects were noted aftercooling.

The coating was then carbonized yielding a satisfactory bonded coatingwith an acceptable surface.

It is understood that the above description of the present invention issusceptible to various modifications, changes and adaptations by thoseskilled in the art, and the same are intended to be considered withinthe scope of the present invention, which is set forth by the appendedclaims.

What is claimed is:
 1. A method for producing an aluminum wettablecathode surface for an aluminum reduction cell, which method comprisesapplying to a cathode substrate a coating composition comprised ofRefractory Hard Material, a thermosetting binder comprising a singleresin selected from the group consisting of phenolic, furane,polyphenylene, heterocyclic, epoxy, silicone, alkyd, and polyimideresins, a mix liquid, carbonaceous filler, and carbonaceous additive,curing said coating composition to produce a relatively hard, adherentsurface layer, and carbonizing said layer to produce an adherentaluminum wettable surface layer consisting of Refractory Hard Materialin a carbonaceous matrix bonded by amorphous carbon, said surface layercharacterized by a percentage of expansion between 800° C. and 1000° C.which differs from the percentage of expansion of said cathode substrateby less than 0.2.
 2. A method as set forth in claim 1, wherein saidcoating composition comprises from about 10 to about 90 percentRefractory Hard Material, from about 1 to about 40 percent thermosettingbinder, from about 2 to about 40 percent mix liquid, from about 1 toabout 60 percent carbonaceous filler, up to about 70 percentcarbonaceous additive, and effective amounts of curing agent and gasrelease agent.
 3. A method as set forth in claim 2, wherein said binderand mix liquid have a char yield greater than about 25 percent.
 4. Amethod as set forth in claim 3, wherein the ablation rate of saidcarbonaceous matrix is essentially equal to the rate of wear anddissolution of said Refractory Hard Material in the environment of saidaluminum reduction cell.
 5. A method as set forth in claim 4, whereinsaid coating composition is applied to carbon cathode blocks external tosaid cell, and cured prior to placement in said cell.
 6. A method as setforth in claim 5, wherein said coating composition is carbonized priorto placement in said cell.
 7. A method as set forth in claim 5, whereinsaid coating composition is carbonized in said cell.
 8. A method as setforth in claim 4, wherein said coating composition is applied to carboncathode blocks external to said cell, and then cured and carbonized insaid cell.
 9. A method as set forth in claim 4, wherein said coatingcomposition is applied to the cathode substrate within said cell, andthen cured and carbonized.
 10. A method as set forth in claim 4, whereinsaid coating composition is applied onto a bonding layer.
 11. A methodas set forth in claim 10, wherein said bonding layer contains acarbonaceous additive selected from the group consisting of carbonaggregate, carbon fiber, and mixtures thereof.
 12. A method as set forthin claim 11, wherein said coating composition further comprises carbonfiber.
 13. A method as set forth in claim 12, wherein said carbon fibercomprises up to about 10 percent by weight of the coating composition.14. A method as set forth in claim 12, wherein said carbon fiber isprepared from a material selected from the group consisting of pitch,rayon, and polyacrylonitrile, and is from about 0.16 cm to about 1.27 cmin length.
 15. A method as set forth in claim 14, wherein said carbonfibers comprise from about 0.05 percent to about 3.0 percent by weightof the composition.
 16. A method as set forth in claim 4, wherein saidcoating composition comprises from about 35 to about 60 percent titaniumdiboride, from about 2.5 to about 8 percent of a thermosetting binder,from about 4 to about 20 percent mix liquid, said binder and mix liquidhavsing a char yield greater than 50 percent, from about 10 to about 40percent carbonaceous filler, about 5 to about 15 percent carbonaceousadditive, and effective amounts of curing agent and gas release agent.17. A method as set forth in claim 16, wherein said carbonaceousadditive includes carbon fiber.
 18. A method as set forth in claim 17,wherein said carbon fiber comprises from about 0.05 percent to about 3.0percent by weight of the coating composition and is a sized fiber.
 19. Amethod as set forth in claim 16, wherein said coating composition isapplied in a plurality of layers.
 20. A method as set forth in claim 19,wherein titanium diboride content changes in successive layers.
 21. Amethod as set forth in claim 16, wherein said coating composition isapplied to an intermediate bonding layer.
 22. A method as set forth inclaim 21, wherein said bonding layer comprises a carbon cement.
 23. Amethod as set forth in claim 22, wherein said bonding layer furthercomprises a carbonaceous additive selected from the group consisting ofcarbon aggregate, carbon fiber, and mixtures thereof.